The Republic
Plato's central dialogue, in which Socrates constructs an ideal city in argument to answer the question of why a just person is happier than an unjust one, producing along the way the Theory of Forms, the Allegory of the Cave, and a political philosophy that has been debated and condemned for 2,400 years.
Summary
The Republic begins with a question about justice: is the just man better off than the unjust man, even if the just man suffers and the unjust man prospers? Socrates answers by building an ideal city in argument, on the grounds that justice will be easier to see at city scale than in one person.
The city he constructs is deliberately totalitarian by modern standards: a strict three-class system (philosopher-kings, warriors, and producers), elimination of private property and family for the guardian classes, censorship of poetry and art that portrays the gods badly, and the Noble Lie, a myth told to citizens about their metallic origins that justifies their class positions. Plato knows these are uncomfortable proposals and has Socrates own them explicitly.
The Theory of Forms underlies the political philosophy: material things are imperfect copies of eternal abstract Forms, and knowledge of the Forms is the only genuine knowledge. Only philosopher-kings, who have ascended from the Cave and seen the Form of the Good, are qualified to rule.
In the Allegory of the Cave, prisoners sit chained facing a wall, seeing only shadows of objects carried behind them, and take the shadows for reality. The philosopher is the prisoner who turns around, is blinded by the firelight, climbs out into sunlight, and is then obligated to come back and govern prisoners who resent the disruption.
Whether Plato endorses what Socrates describes or uses it as a reductio ad absurdum of the question about justice remains genuinely contested.
Quotes
"The price good men pay for indifference to public affairs is to be ruled by evil men."
"The beginning is the most important part of any work."