Prince Myshkin arrives in Petersburg to find whatever family or connections he may still have in Russia. Immediately he stumbles on the sad story of the very beautiful Nastasya Philipovna, whose time as a kept woman has ruined her life and made her half-crazy. Myshkin falls in pity-compassion-love with her from the first time he sees her portrait.
The very night he meets her in person, Myshkin proposes to Nastasya and she takes him seriously. After she flees from Rogozhin, she runs away to live with Myshkin. At the same time, a will comes to light that leaves Myshkin a sizable fortune.
Because his love for Nastasya isn't really romantic love, she flees from him. Because he is so innocent and unable to understand the underlying motivations of people's actions, he is systematically defrauded and conned out of much of his money. Rogozhin, Nastasya's other suitor, threatens to kill him out of jealousy. And finally, he meets Aglaya, and actually falls in love for the first time.
Although he is now engaged to Aglaya, Myshkin is haunted by Nastasya, his commitment to her, and his deep compassion for her suffering. After Aglaya and Nastasya come face to face, Myshkin abandons Aglaya and again is engaged to Nastasya. Everyone around him is against the idea of this wedding. The preparations go on though, even in the face of Nastasya's fits of rage and his own feelings of despair.
The day of the wedding, Nastasya runs off with Rogozhin, who takes her as a willing victim back to his house and kills her. When Myshkin finds them, he spends a long and horrible night with Rogozhin. At the end of this ordeal, he has lost all of his mental faculties and reverts to a near-catatonic state.