Socrates visits the Piraeus, where successive interlocutors offer definitions of justice — paying debts, helping friends and harming enemies, the advantage of the stronger — all of which Socrates dismantles through dialectical questioning.

Glaucon and Adeimantus demand that Socrates prove justice is good in itself. Socrates proposes building an ideal city to see justice 'writ large,' then designs a rigorous education for the guardian class, censoring poetry that misrepresents gods or encourages vice.

Socrates completes the city's structure, identifies justice as each class performing its proper function, and argues the soul has three corresponding parts — reason, spirit, and appetite — with justice consisting in their proper harmony.

Socrates introduces three radical proposals — equality of women, communal families, and philosopher-kings — then argues that only those who grasp the Forms (true reality beyond appearances) are fit to rule, culminating in the Allegories of the Sun and the Divided Line.

Socrates presents the Allegory of the Cave — prisoners chained in darkness who mistake shadows for reality — then outlines the mathematical and dialectical education that liberates the philosopher from the cave and prepares them to rule.

Socrates traces the decline of political constitutions from aristocracy through timocracy, oligarchy, democracy, and tyranny, with each regime mirrored by a corresponding soul-type. The tyrannical soul is revealed as the most wretched, proving justice's superiority.

Socrates returns to the censorship of poetry, arguing that imitative art appeals to the irrational soul and must be banished. He then offers proofs of the soul's immortality and concludes with the Myth of Er — a vision of cosmic justice where souls choose their next lives.