Apollodorus tells to his friend a story of a symposium, or banquet, that was hosted by the playwright Agathon to celebrate his victory in a dramatic festival the night before. Socrates is late to arrive, after he became lost in thought on the porch of a nearby neighbor. When they are done eating, Eryximachus takes the suggestion made by Phaedrus, that they should all make a speech in praise of Eros, the god of love and desire. It will be a competition of speeches to be judged by Dionysus. It is anticipated that the speeches will ultimately be bested by Socrates, who speaks last.
Phaedrus starts by pointing out that Eros is the oldest of the gods, and that Eros promotes virtue in people. Pausanias is next, and he contrasts common desire with a “heavenly” love in the form of pederasty, which occurs between a man and a young boy; and in which the boy gives sexual pleasure, and in return gains knowledge and virtue. Next Eryximachus speaks, and suggests that Eros encourages “sophrosyne”, or soundness of mind and character, and is not only about human behavior, but also occurs in music, medicine, and many other areas of life.
Next is Aristophanes, the comic playwright. Aristophanes tells a fantastical, mythological story about how humans were at one time twice the people that they are now, but this was seen as threatening to the gods, so Zeus cut everyone in half. And ever since, humans go about in search of their other half, in order to become whole. Agathon, the host, follows Aristophanes, and his speech sees Eros as youthful, beautiful, and wise; and as the source of all human virtues.
Socrates asks questions of Agathon: has he referred to the object of love, or love itself? Socrates then relates a story he was told by a wise woman called Diotima. According to her, Eros is not a god, but is a spirit that mediates between humans and their objects of desire. Love itself is not wise or beautiful, but is the desire for those things. Love is expressed through propagation and reproduction: either physical love or the exchanging and reproducing of ideas. The greatest knowledge, Diotima says, is knowledge of the “form of beauty”, which humans must try to achieve.
When Socrates is nearly done, Dionysus, who is to judge the speeches, arrives, in the person of Alcibiades. Alcibiades crashes in, terribly drunk, and delivers a encomium to Socrates himself. No matter how hard he has tried, he says, he has never been able to seduce Socrates, because Socrates has no interest in physical pleasure.
Under the influence of Dionysus, who is now in attendance, the party becomes wild and drunken. Aristodemus goes to sleep. When he wakes up the next morning Socrates is still talking and debating. Soon everyone at last falls asleep, as Socrates rises up and goes off to tend to his daily business as usual.
In order to understand Plato more clearly, it is useful to bear in mind his theory of Forms, according to which all the phenomena perceived by the senses are imitations of eternal and perfect Forms that alone have reality. Beauty belongs to this category of Forms.