The framing for this sutta takes place during one of the Brahmin Uposatha fast-days.1
On this sacred moonlit night, King Ajātasattu - the ruler of the kingdom of Magadha - found himself in his feelings on the roof of his palace. He told his ministers that the night's beauty had moved him to seek the counsel of a holy man, but his hope that the holy man could bring peace to his heart perhaps betrayed what was really on the king's mind; the guilt of having arranged the death of his father, the good King Bimbisāra, in order to seize the throne.
His ministers listed six such holy men who were in the area, but the king remained silent after each suggestion. He had previously met with all of these men, and none of them had impressed him. The king's royal physician, Jīvaka Komārabhacca, was also nearby, and he had not yet spoken. The king questioned him, and he said that there was another holy man, not yet mentioned. This was the Buddha, and he was staying in Jīvaka's mango grove with around 1,250 monks.2 Jīvaka suggested the king go and meet him. The king thought this was a great idea, and he told Jīvaka to ready 500 elephants. The king, his 500 wives, some torchbearers, and Jīvaka himself set out for the mango grove in order to meet the Buddha.
As they approached the peaceful grove, the king grew suspicious; if there were over 1,000 monks staying here, how could the place have been as quiet as it was? The silence disturbed him, and he asked Jīvaka if he was being led into a trap. Again, the king's paranoia perhaps betrayed his guilt. Jīvaka assured him everything was okay, and pointed out the Buddha. As the king approached, the tranquilty inspired him, and instead of being frightened, he commented aloud, "if only my son, Prince Udāyabhadda, could be this peaceful..." The Buddha replied directly, "Do your thoughts go to someone you love, king?"
The king and the Buddha spoke for a moment, then the king got to the point: he had a question he would like the Buddha to answer. The Buddha told him to ask his question freely. "There are so many people who live in so many different ways," the king began, "and they enjoy the fruits of their labor here-and-now, in this lifetime. Tell me, why would anyone become a homeless religious devotee? What are the benefits, which can be seen here and now, of following your path?"
The Buddha assured the king that he would answer the question, but first, he wished to ask the king a question: "Have you asked any other ascetics or brahmins this question?" The king responded that he had, in fact, asked other religious teachers this question before: he had brought this question to the six men his ministers had previously recommended visiting. The Buddha asked him how these six men answered.
The king began by recounting how he posed this question to Pūrana Kassapa, who taught the doctrine of "non-action" (or what we may now call Amoralism). Pūrana Kassapa told him that, ultimately, there is no such thing as good or evil - even if someone were to deliberately kill every living being on the planet, no evil has been committed, and nothing bad would result from such an act. Likewise, there is no good deed that can be said to produce merit.
The king tells the Buddha that he was incredibly unsatisfied with this answer, but he did not feel it was his place to question a revered ascetic - after all, the king had his own father's blood on his hands - so he humbly left Pūrana Kassapa in silence. The king did the same with the other teachers, as well, so for the sake of brevity I won't repeat these parts below.
Next, the king recounts his encounter with Makkahali Gosāla. This man was the leader of the Ājīvikas, a religious group from the Buddha's time. They seem to have been quite influential, given that they are mentioned by name alongside Buddhists and Jains in later sources, but they died out some time in the Common Era. It is difficult to say exactly what their beliefs were, as their doctrines are mostly known through Buddhist and Jain records (such as this very sutta). These sources, obviously, depict the sect in a bad light. It seems they wandered around naked, and they were forbidden from eating food in many circumstances, leading to an extremely sparse diet. They were also described as disregarding social etiquette, with one specific example being that they licked their fingers and hands clean after those rare instances when they did eat.
Makkahali Gosāla said a great many things in response to the question, some of which were finely-detailed ontological or cosmological claims. Most relevant is the fact that he told the king that we are ultimately powerless in the world - everything is pre-determined. There is no such thing as causality, no such thing as conditioning or dependent origination. Beings have no free will, and people are deluded or enlightened as a mere matter of circumstance, regardless of their own intentions and actions. He described people on the spiritual path like a ball of yarn which is set rolling; it will simply unwind when the time comes. Just like the ball of yarn can take no action to speed up or slow down its unraveling, people cannot affect their own progression towards enlightenment.
The next man the king questioned was Ajita Kesakambalī, or "Ajita Who Has a Garment of Hair." He earned this name by wearing a cloth made out of human hair, and he was well-known in the Buddha's time. Like the other two men, he claimed that actions did not produce any fruits, either positive or negative. He denied the existence of any afterlife, rebirth, or meditative attainments. He claimed that charity is a fools' endeavor, because ultimately humans are merely material beings, and at the moment of death, both the fool and the wise man are annihilated all the same.
This next man was Pakudha Kaccāyana, the founder of a type of eternalist philosophy called "Atomism." He told the king that reality consists entirely of seven great substances: Earth, Water, Fire, Wind, Pleasure, Pain, and Soul. These substances are eternal, having never been created and never being destroyed - they have always been, and will always be. Everything in reality is simply the movement of these elements, and on an ultimate level, these substances do not interact with or obstruct one another. Even if you were to cut off someone's head, ultimately, no one is dying, and no one is committing any evil - it is a mundane act of the blade moving through the space between these seven substances.
The king had also brought his question to Nigaṇṭha Nātaputta, perhaps better remembered by history as Vardhamāna Mahāvira, the founder of Jainism. He was a little older than the Buddha, and certainly proved to be the most enduring of these rival teachers, as Jainism is still practiced today.
He told the king that a Nigaṇṭha - the word he and his followers used to refer to themselves, meaning "free from bondage" - has four restraints which keep him perfectly self-controlled. It's difficult to make sense of what, exactly, these four restraints are; part of this is due to the obscurity of the Pali word "vāri," which can mean "water" or "restraint."
Bhikku Sujato's translation renders the passage as follows: a Jain ascetic practices the four restraints by being "obstructed by all water, devoted to all water, shaking off all water, pervaded by all water."
Both Sujato and Maurice Walshe note that it's difficult to say whether or not this passage reflects Jain philosophy at the time; it does not correspond directly to any known Jain text, but Jain ethics do include rules regarding water (meant to protect aquatic life). Likewise, they both note the possibility of this passage being satirical.
Sujato errs on the side of caution and assumes that the passage refers to actual Jain practices which would have been well-known at the time. Specifically, he believes "obstructed by all water" refers to the way Jains forbade swimming in rivers or crossing bodies of water in ways that could harm aquatic life. Next, "devoted to all water" refers to the way that Jains were devoted to protecting the life of creatures which live in water. "Shaking off all water" could be a poetic way of portraying renunciation of worldly things, or more specifically, a rule forbidding Jains from drying their bodies using towels or heat. Finally, "pervaded by all water" might refer to the way that the human body contains water along with a multitude of organisms that inhabit it.3
Walshe, on the other hand, states confidently that the passage is simply a piece of satire at the expense of the Jains. He translates "vāri" as "curb" (or restraint), but ultimately concludes that the exact translation of the word is less important than the apparent contradiction. The point is to demonstrate how absurd it is that a Nigaṇṭha - one "free from bondage" - is restrained in four different ways.4
I won't draw a conclusion here - to do so would be beyond my understanding. Let us, instead, continue the story... the king was unhappy with this answer.
Finally, the king consulted Sañjaya Belatthaputta5, who was one of the extreme agnostics whom the Buddha called "eel-wrigglers" (see DN 1). In classic eel-wriggling fasion, Sañjaya simply responded by doubling down on his commitment to never consider or answer any sort of "big-picture" philosophical or cosmological issue. Amusingly, the king noted that this man was the stupidest of all the ascetics he had questioned, demonstrating how critical Buddhism was of "eel-wriggling" agnostics.
The king repeated his question to the Buddha: "What are the benefits, which can be seen here and now, of renouncing the world?" Once more, the Buddha assured the king that he would answer the question, but he wished to pose another question to the king first.
The Buddha asked the king to imagine a scenario involving one of his slaves. This slave reflects on how both he and the king are mortal men, but the king enjoys pleasures like a god while he toils away. He thinks about meritorious deeds, and how the kammic fruits of such deeds can lead can to different rebirths in different stations of life. This slave then decides to renounce the worldly life, becoming a homeless religious practitioner. He becomes content, living off of simple food and living in simple shelter. He experiences delight in his newfound solitude.
Having put forward this hypothetical, the Buddha asked if the king would demand this former slave, now liberated by his religious lifestyle, to return to servitude. The king responded, "Of course I wouldn't - renunciates must be treated with the utmost respect, and have their necessities provided for them."
The Buddha concluded by convincing the king that this was one fruit of renunciation; it was a means of liberation from class inequality, and provided an opportunity for those under the weight of societal expectations to live a happy life free from any such burdens. During the Buddha's time, renouncing the life of a householder and joining a religious order was a means of social liberation for many lower-class people.6
The king asked the Buddha for more examples. This passage is almost exactly the same as the previous one, only with the Buddha's hypothetical involving a farmer employed by the king instead of a slave. Once again, the king asked for another example, this time "more excellent and perfect."
This next section is actually quite lengthy, listing many different things, though the Buddha only concluded with the previously-established statement "This, great king, is a fruit of renunciation..." after discussing the first jhāna. I have attempted to organize this summary in a way which reflects the structure of the sutta.
He declared that, eventually, a fully-enlightened Buddha arises in the world, and goes forth to teach the Dhamma. This set the stage for another hypothetical: he asked the king to consider a man who hears this message and decides to join the Buddha's order. This man, now a monk, lives according to the monastic code and becomes "accomplished in ethics." He "guards the sense-gates." He is mindful, alert, and content. The following subsections consist of the Buddha's explanations of these statements.
This subsection segues into the three categories of ethics, repeated from DN 1. For the sake of brevity, I will not reproduce them in their entirety here. In short, they are ethical practices observed by the Buddha's followers which distinguished them from the "ascetics and brahmins" who belonged to rival groups in the Buddha's time.
The Buddha used them to explain what it meant for his hypothetical monk to be "accomplished in ethics." The Buddha told the king that a monk accomplished in ethics is free from the dangers of immorality, just as a ruler who has conquered his enemies is no longer threatened by them. As a result of his morality, he experiences "blameless happiness" inside himself.
Guarding the Sense-Gates
The Buddha explained to the king that "guarding the sense-gates" means practicing restraint during the sensory process.7 When one of the sense-gates becomes engaged by a sense-object, and sense-consciousness arises, someone who is practicing restraint is able to process the experience without becoming fascinated with the "signs" of that sense-object. Someone who is unrestrained during this process, however, may become infatuated with the qualities of whatever thing has engaged their senses, and in this way they may become overwhelmed by desire for (or aversion toward) that sense-object.
A monk who is restrained in his senses is able to experience "unsullied happiness" inside himself.
Being Mindful and Alert
The Buddha explained that a monk always acts mindfully and with alertness, no matter what it is they're doing. They don't allow themselves to get lost in daydreams and they don't let their mind wander. They control their minds when doing anything and everything.
A monk is content with the bare essentials: his robes, and alms donated to him as food. He is as free a bird, unburdened by unncessary possessions.
Abandoning the 5 Hindrances
Having become established in ethics, guarding his sense-gates, mindful and alert during all activities, and having contentment, our hypothetical monk is now ready to go to some secluded place and practice meditation. Legs crossed, body straight, he is able to abandon the 5 Hindrances - states of mind which act as obstacles to enlightenment and inhibit wisdom (SN 46.37).
He abandons desire by giving up longing for worldly things. He abandons ill-will by meditating with compassion for all living beings. He abandons sloth-and-torpor by being mindful and alert, "meditating on the perception of light" - visualizing the brightness of daylight (see AN 4.41 & 7.58). He abandons restlessness-and-worry by calming the mind. Finally, he abandons doubt by being certain about what sorts of things are skillful.
Having abandoned the 5 Hindrances, a monk attains freedom, like a debtor who has paid his debt, or a sickly person who has recovered, or a prisoner released from jail. During his meditation, he becomes joyful, and from this joy, a state of mental delight arises. With the mind feeling delighted, the body grows tranquil. From this tranquility, they experience bliss. From this bliss, he begins to become immersed in "samādhi," a profound quality of concentration, which leads to transition into the first jhāna.
The First Jhāna
In the 1st jhāna, one now experiences rapture and bliss which arise through the absence of sense-desires as well as the absence of any unskillful mental states. In this stage, he is still using his discursive, thinking mental faculties to gently guide the mind and contemplate the experience of the first jhāna. Although this should be understood as a type of thinking, it is important to understand that this is much more refined than the mind’s usual process of thought—there is no mental chatter, no train of internal dialogue. Having been freed from the unskillful mental states that cloud our usual thoughts, the mind is now functioning in a much more subtle, productive way.
The Buddha explained to the king that, in the first jhāna, a monk's body - head to toe - is completely filled with this rapture and bliss. He concluded by saying this is another fruit of renunication.
Eventually, he is able to bring even this subtle process of thinking to a stop. This leads to entry into the 2nd jhāna. Rapture and bliss now arise from a more refined state of samādhi. The mind becomes "one-pointed," completely focused, and a powerful state of confidence and mental clarity arises.
Once again, the Buddha explained that a monk's body is completely filled with this new kind of rapture and bliss. He told the king that this was another fruit of renunication.
In the third jhāna, the previous feeling of "rapture" fades away, leaving a monk with "bliss free from rapture" which fills and permeates his entire body. While meditating, he experiences equanimity and mindful awareness.
The Buddha told the king that this "bliss free from rapture" was another fruit of renunciation.
In the fourth jhāna, a monk goes beyond all feelings, even the previous jhānic feeling of bliss which had permeated his body. He has "given up all pleasure and pain," and his mind experiences the purity of equanimity and mindfulness. The Buddha explained this as a "pure, bright awareness" which fills the entire body.
After having progressed through these four jhānas, a monk's mind is "immersed in samādhi," a state of sublime concentration. In this state, his mind is purified in such a way which enables him to direct it toward "knowledge and vision." In doing so, he attains direct insight into the teachings of the Buddha.
Here, the Buddha specifically used the example of the body and consciousness; he told the king that a monk discerns that his body is a physical thing, made up of the four great elements, born from sexual union, sustained on food, impermanent, and will eventually be destroyed. He also discerns that his consciousness is "supported by and bound to" his body.
It should be noted that this is not a mere intellectual understanding - it is a direct experience of this reality. The Buddha told the king that this insight is another fruit of renunciation.
A monk immersed in samādhi may also direct the mind to the creation of a "manomaya-kaya," or a "mind-made body." In Buddhism, the belief is that a highly-advanced meditator is able to literally produce another body - complete with limbs and faculties - through the power of their purified mind. This is described like pulling a sword from its scabbard; the sword comes from its sheath, but they can both exist separately and be used apart from one another. This mind-made body can be used to visit other realms of existence, or simply to be in two places at once here in the mundane world.8
The Buddha told the king that this was yet another fruit of renunciation.
Immersed in samādhi, a monk may also direct his mind to a number of different supernatural feats. He can multiply himself, pass through solid barriers, descend into the earth and return to the surface (as if it were water), walk on water (as if it were earth), fly through the air while sitting cross-legged, "touch and stroke the sun and the moon,"9 and travel as far as the Brahmā realm.
The Buddha told the king that the ability to perform feats such as these is another fruit of renunciation.
In addition to the above abilities, a monk immersed in samādhi may become capable of clairaudience (supernatural hearing). This allows him to hear sounds both near and far, in both the human and divine worlds. The Buddha said this is another fruit of renunication.
Another supernatural feat gained in samādhi is the ability to read minds. The Buddha describes this less like the usual understanding of mind-reading, where you hear someone's internal monologue, and more like a sort of empathy which allows you to understand someone's mental state - detecting whether or not someone's mind has greed, hatred, delusion, or various qualities associated with spiritual attainment. The Buddha said this is another fruit of renunciation.
A monk immersed in samādhi may recollect their past lives. This recollection may span innumerable eons back, and include a great level of detail: names, clans/families, the pleasures and pains experienced, and the ways they died. The Buddha said this is another fruit of renunciation.
When his mind is immersed in samādhi, a monk may use the "divine eye" (dibbacakkhu) to discern how other beings die and are reborn according to their kamma (more commonly known by the Sanskrit word, karma). He can see these beings rising and falling from the various realms of rebirth, and he understands the way the fruits of their volitions cause these states of existence. This, the Buddha told the king, is another fruit of renunciation.
Finally, while immersed in samādhi, a monk may apply his mind to the knowledge of destroying the corruptions (āsavās - tainted mental states which lead us to rebirth). In the suttas, the corruptions are usually three in number: 1- The weakness for sensual pleasure (kāmāsava). 2- The weakness for continued existence (bhavāsava). 3- The weakness for ignorance (avijjāsava).
Someone whose mind is still under the power of the corruptions is made vulnerable to the allure of sensual pleasure, continued existence, and ignorance. The corruptions subtly work beneath the surface and influence the way a person behaves. Destroying them is equivalent to enlightenment and the attainment of nibbana. A monk is able to achieve this while immersed in samādhi.
He is able to directly comprehend the Four Noble Truths: he gains an experiential understanding of dukkha, its origin, its cessation, and the Eightfold Path which leads to its cessation. Likewise, he gains an experiential understanding of the corruptions, their origin, their cessation, and the practice which results in their destruction. Having done so, he has become enlightened - his journey has come to an end, and he has escaped the cycle of rebirth. Upon death, no continued existence will follow.
The Buddha told the king that this is the greatest fruit of renunciation.
After the Buddha finished speaking, King Ajātasattu was delighted; finally, he had received an answer to his question. On the spot, he declared himself a lay follower of the Buddha. Suddenly, he confessed his guilt, admitting that he had his father killed in order to steal his crown. Of his own accord, he admonished himself for being foolish and immoral. The Buddha agreed, but accepted his confession, since he acknowledged his error and had resolved to behave properly in the future.
As we will see throughout the canon, whether or not Ajātasattu was truly reformed could be a matter of debate. The king's hesitance to fully commit is betrayed here by the fact that the king immediately rose up, leaving to tend to the worldly concerns which pressed on him. He did so respectfully, however, paying all proper respects to the Buddha and observing decorum - he bowed, circumabulated the Buddha with the right of his body towards the Buddha, and then took his leave.
After the king had gone, the Buddha told his monks that the king was "broken." He explained that the king could have attained the "Dhamma eye" (dhammacakkhu)10 after receiving this teaching, if only he didn't bear the weight of his father's death. This concludes the Samaññaphala Sutta.
The corresponding sutra from the Chinese Canon is Dīrghāgama 27. Unfortunately, I don't have access to an English translation of DA 27; I am aware of one which exists in print form, but the collection to which it belongs is quite expensive and widely criticized for being a poor translation.
I may update this section if I ever gain access to a translation of DA 27.
 "Uposatha" was a word used for holy days in various traditions during the Buddha's time, and would come to be used by the Buddha and his sangha to refer to days observed by both monastics and lay followers. Lay followers simply observe more precepts more intensely, allowing them to "emulate the arahants" for a day (as described in the Muluposatha Sutta). Monks gather and confess if they have violated any of the monastic codes, then recite the rules together. Uposatha days are determined by a lunar calendar. The sutta tells us this particular story took place on the 15th day (Komudi) of the 4th month (Kattika), on the night of a full moon. In his translation of the DN, Maurice Walshe notes that this corresponds to mid-October to mid-November in our own calendar.
 It was common for wealthy people to donate some of their land to be used for holy men and their followers, either as a temporary shelter while traveling, or as a permanent establishment for teachers to whom they were specifically devoted. This was seen as a noble, charitable thing for wealthy people to do.
 See "Long Discourses of the Buddha," translated by Maurice Walshe, Sutta 2, note 115
 It's never explicitly stated in the suttas, but Buddhist history treats Sañjaya Belatthaputta and Sañjaya Parabajjaka as one and the same.
 Even in India today, Buddhism remains a powerful vehicle for class struggles among the socially disenfranchised. This is most evidently embodied in the movement inspired by Dr. B.R. Ambedkar.
 For a more detailed explanation of this sensory process, and its importance in Buddhist philosophy, read my summary of the Buddha's basic teachings here.
 It's possible that this idea is shocking to you. Many people who discuss Buddhism try to downplay more fantastic beliefs such as this, particularly those who are interested in portraying Buddhism as some kind of ancient psychotherapy. The simple truth is this: Buddhism is a religion, and it is filled with religious ideas and beliefs. It should not be held to the standards of a science.
 Walshe notes that this line likely refers to some kind of psychic experience, and isn't meant to be taken literally. However, given that the other supernatural feats presumably are meant to be taken literally, a direct reading isn't exactly out-of-place.
 Rhys Davids, referenced in Walshe's translation, notes that this is one of the supernatural powers of "vision," similar to the previously mentioned "divine eye." There are three such powers, beginning with the lowest, the divine eye. The Dhamma-eye is the middle power, and the highest is the "wisdom eye" (paññacakkhu).