The Idiot opens on a train into St. Petersburg, where Prince Lef Nicolaievitch Muishkin, returning from years of treatment for epilepsy in Switzerland, falls into conversation with the dark, passionate merchant's son Rogojin. The prince is open, unguarded, and so plainly without guile that his fellow travellers take him for a simpleton. He carries almost nothing, knows almost no one, and steps into Russian society with a distant relation, the Epanchin family, as his only foothold.
His goodness immediately unsettles people. Where others calculate, he tells the truth. Where others judge, he forgives. He becomes entangled at once with Nastasia Filippovna, a beautiful woman whose past as a rich man's mistress has filled her with shame and self-contempt, and whom several men are bargaining over like a possession. The prince offers to marry her not out of desire but out of pity, seeing her injury rather than her scandal.
Two loves pull at him through the book. One is Nastasia, broken and half-mad, who keeps fleeing toward Rogojin and the ruin she half-wants. The other is Aglaya Epanchin, young, proud, and quick, who is drawn to the prince's strange purity but cannot bear to share him. Muishkin, unable to wound either woman, is torn between them, and his very inability to choose or to hurt anyone helps drive the disaster.
Around this triangle Dostoevsky crowds a loud cast of schemers, drunkards, dying young cynics, and respectable families anxious about reputation and money. The prince absorbs their cruelty without returning it, and again and again his candor lays bare what the others hide. He speaks against capital punishment, doubts the hardness of unbelief, and treats children and the disgraced with the same gentleness he gives to generals.
The book ends in catastrophe. Nastasia abandons her wedding to the prince and runs to Rogojin, who kills her. The two men keep vigil through the night beside her body, and the strain undoes Muishkin entirely. He relapses into the helpless condition he came from, so that the doctor who first treated him would again call him an idiot. His goodness has changed no outcome and saved no one, and the question of what such a man is for is left open and raw.