The action takes place in ancient Greece. We meet with the main character at a difficult moment in his life: expelled from his native city - Athens, Agaton heads to the Middle East. Lost in the mountains of Thrace, he accidentally falls on the feast of Bacchus, which is celebrated by the noble inhabitants of this area. Cilician pirates suddenly attack the participants in the celebration and lead them into slavery. Among the captives is Agaton. On the ship, he meets a girl named Psichet, whom he was in love with while still living in Delphi, and with whom he was forcibly separated. She manages to tell him how she was sent to Sicily. There, learning that Agaton in Athens, dressed in a man’s dress, she runs, but along the way she falls into the hands of pirates, who now, like Agathon, will sell her into slavery.
At the slave market in Smyrna, a handsome, educated young man is bought by the rich sophist Gippias, who is going to make him his student and philosophical follower. Kallias, as he calls Agathon, is an adherent of the philosophical teachings of Plato. The desire for refined pleasures is alien to him, he feels uncomfortable in the house of Gippias with his far-fetched morality. In long dialogues and monologues, Gippias tries to convince the young man that the main thing in life is to satisfy his needs. The art of being rich is built on the ability to subjugate the property of other people, and so that it looks like a voluntary act on the part of these people.
All the efforts of Gippias do not lead to anything, then he acquaints his stubborn slave with the charming heterosexual Danae, hoping that she will be able to persuade Agaton with her love to his side. At first, a beautiful getter only pretends to be a virtuous and responsive lover, but gradually the sincerity of the young man, his devotion give birth and there is a real feeling in her.
Danae Agathon tells the story of his life. He grew up in Delphi at the temple of Apollo, he was destined for the fate of the priest. He sincerely believed his mentor Theogiton, but he deceived him. Once he played Agathon, appearing in front of him in the Nymph grotto in the image of Apollo, when the student revealed fraud, he began to explain that "everything that was said about the gods was an invention of cunning heads." Agaton befalls a terrible disappointment, but he succeeds in not losing his final faith in "the highest spirit." His own philosophical considerations give him strength. So he reaches the age of eighteen when the middle-aged high priestess Pythia falls in love with him. She seeks his love, Agaton at first, by her naivety, does not understand her intentions. One of the priestess's slaves was Psichet, a girl who, at the age of six, was abducted from Corinth by robbers and sold into slavery at Delphi. Agaton falls in love with Psisch, their soul mates are drawn to each other, they begin to secretly meet at night near the city in the grove of Diana. But the jealous mistress of the girl learns about the tendency of young people to each other, she comes on a date instead of Psichet. The young man rejects the love of Pythia, and then the humiliated priestess sends the slave to Sicily.
Agathon flees Delphi in search of Psichet. In Corinth, he meets his father, who recognizes a young man on a city street by similarities with his deceased mother. Stratonik, the name of the father of Agathon, is one of the most notable inhabitants of Athens. Since Agaton, like his younger sister, was born out of wedlock, he sent him to Delphi so that he could receive a decent upbringing and education at the Temple of Apollo. Where is his younger sister now, he does not know.
Together with his father, Agaton settles in Athens and becomes the legal citizen of the republic. The father soon dies, leaving his son the only legitimate heir. Agaton is studying at the philosophical school of Plato. He stands up for his unjustly accused friend, which attracts the discontent of some wealthy Athenians. The young man seeks to destroy the distinction between rich and poor in the republic, advocating the return of the "golden age". Gradually, through his activities, he makes his enemies who declare Agathon a state criminal and expel them from Greece. So he eventually finds himself in the house of Gippias.
The love of Danai and Agathon is not included in the plans of the prudent sophist, and he destroys the idyll, telling Kallias about the dubious past Danai. In desperation, Agathon flees from Smyrna, he goes to Syracuse, where, according to rumors, the young tyrant Dionysius became an enthusiastic student of Plato, the Young man hopes to find use of his forces there.
After a detailed description of the relationship at the court in Syracuse, the author returns to the story of his hero. Agaton meets in the city of the philosopher from Cyrene, Aristippus. His worldview combines gaiety of temper with peace of mind. This wise man represents Agathon at the court of Dionysius. Soon, an educated young man becomes the first adviser to the tyrant. For two years, Agathon softens by all means available to him the oppression of Dionysius against the people. He indulges the tyrant’s minor weaknesses in order to overcome his much more serious flaws. The people of Syracuse venerate Agathon as their protector, but, on the other hand, he makes his enemies among the courtiers. The former Minister Filistus and the former favorite Timocrates hate him. In addition, Agaton finds himself involved in the court intrigue of the intelligent, beautiful and power-hungry wife of Philistus Cleonissa, whose love he rejects, while Dionysius solicits her. Anticipating a fatal outcome, Aristippus advises Agathon to leave, but the maelstrom of events captures a passionate young man. He becomes a participant in the conspiracy of the exiled brother-in-law of Dionysius, Dion. Philistus reveals the plot, and Agathon is arrested.
In prison, the hero’s philosophical views are severely tested, from a champion of virtue and an intercessor, he is ready to turn into an embittered man-hater. The unexpected arrival of Gippias in Syracuse sobering Agathon. He again refuses to accept the sophist's offer to become his follower in Smiryan and finally decides to always wish people only good and do good only. The famous statesman, philosopher and commander Architent of Tarenta frees Agaton.
In Tarentum, the hero finds his new home. The architect, who knew Stratonik well, replaces his father. Here, Agathon finds his beloved Psichet, who became the wife of the son of Architus, Critolaus, and finds out that she is actually his own sister.
Agaton in Tarentum delves into the study of sciences, especially the natural ones. Once during a hunt, he finds himself in a secluded rural house, where he meets Danae, who calls herself Haricley. With a story-confession about her life, she acquires a faithful friend in the person of Agaton. Psichet becomes her friend.
The architect with his life wisdom crowns the spiritual formation of the protagonist of the novel. The political successes of the practical philosophy of the Tarent activist make a strong impression on Agaton. During the thirty-year reign of Architus, the inhabitants of Tarenta are so accustomed to the wise laws of their ruler that they do not perceive them otherwise than as something natural and ordinary.
After traveling around the world in order to learn as much as possible about the life of other peoples, Agaton devotes himself to Tarentum social activities. He sees the meaning of his life in achieving the prosperity of this small state with its well-meaning inhabitants.
