Prince Myshkin is an epileptic returning from a sanitarium. On the train, he meets Rogozhin and they become friends. Myshkin visits his distant relatives, the Epanchins, a fashionable family. General Epanchin gives him a job and he fascinates Madame Epanchin and her daughter, Aglaya, with his innocence and awkwardness.

The Prince boards with Ganya, a schemer who wants to marry Aglaya for her money. Ganya is also involved with Natasya; though innocent, she is a kept woman. Myshkin pities Natasya; in their innocence they are two of a kind. He offers to marry her, but as she is worried about ruining his name, she runs off with Rogozhin. Shortly afterward, she runs away from Rogozhin and disappears. Rogozhin assumes she has run to Myshkin and with Ganya plots the Prince's death.

Meanwhile, Aglaya has fallen in love with Myshkin, but his bizarre talk disturbs the family and when he falls into a fit at a party they ban him from the house. Aglaya also grows increasingly jealous of Natasya. The two women meet and Aglaya resolves to give up the Prince. At last, Natasya agrees to marry Myshkin but on their wedding day, she elopes with Rogozhin, who murders her. Myshkin returns to the sanitarium.