In the remote Peloponnesian township of Phlius, Echecrates encounters Phaedo of Elis, one of the men present during Socrates' final hours. Eager to hear the story from a first-hand source, Echecrates presses Phaedo to tell what happened.
A number of Socrates' friends were gathered in his cell, including his old friend Crito and two Pythagorean philosophers, Simmias and Cebes. The account begins with Socrates proposing that though suicide is wrong, a true philosopher should look forward to death. The soul, Socrates asserts, is immortal, and the philosopher spends his life training it to detach itself from the needs of the body. He provides four arguments for this claim.
The first is the Argument from Opposites. Everything, he says, comes to be from out of its opposite, so that for instance a tall man becomes tall only because he was short before. Similarly, death is the opposite of life, and so living things come to be out of dead things and vice versa. This implies that there is a perpetual cycle of life and death, so that when we die we do not stay dead, but come back to life after a period of time.
The second is the Theory of Recollection. This theory suggests that all learning is a matter of recollecting what we already know. We forget much of our knowledge at birth, and can be made to recollect this knowledge through proper questioning. That we had such knowledge at birth, and could forget it, suggests that our soul existed before we were born.
The third is the Argument from Affinity. Socrates draws a distinction between those things that are immaterial, invisible, and immortal, and those things which are material, visible, and perishable. The body is of the second kind, whereas the soul is of the first kind. This would suggest that the soul ought to be immortal and survive death.
At this point, both Simmias and Cebes raise objections. Simmias suggests that perhaps the soul is like the attunement of a musical instrument. The attunement can only exist so long as the instrument exists, and no longer. Cebes admits that perhaps the soul is long-lived, and can outlive many bodies, but argues that this does not show that the soul is immortal.
Socrates replies to Simmias by pointing out that his theory of attunement is in conflict with the Theory of Recollection, which proposes that the soul existed before the body. As for Cebes, Socrates embarks on a complex discussion of causation that ultimately leads him to lay out his fourth argument, positing the unchanging and invisible Forms as the causes of all things in this world. All things possess what qualities they have only through participation in these Forms. The Form of Life is an essential property of the soul, Socrates suggests, and so it is inconceivable to think of the soul as ever being anything but alive.
Socrates concludes with a myth of what happens to souls after death. Then he has a bath, says some last goodbyes, drinks the poisonous hemlock, and drifts imperceptibly from this world to the next.