After spending several years in a Swiss mental institution, where he was treated for epilepsy, Prince Lyov Myshkin returns penniless to Petersburg and visits distant relatives — the Epanchins. He proves himself a master of calligraphy and is hired by General Epanchin, who finds that he is strangely fond of the prince. Myshkin is then introduced to the general's wife and daughters and, afterward, he goes home with Ganya Ivolgin, the general's secretary; Myshkin will board with the Ivolgins.
At Ganya's, Myshkin meets the Ivolgin family and quickly finds himself involved in a family quarrel when he mentions that Ganya has received a portrait of Nastasya Filippovna. Ganya's family is fearful that Ganya will marry Nastasya — a beautiful, but well-known "kept" woman. In the midst of the quarreling, Nastasya appears. She is as bewitchingly beautiful as Rogozhin (a fellow Myshkin met on the morning train) said she was; Myshkin immediately understands Rogozhin's sworn devotion to her. And Nastasya is as striking as the portrait which Ganya showed earlier in General Epanchin's office.
Nastasya is polite to the Ivolgins only momentarily. She turns surly and is sarcastic to Ganya's mother and sister and then turns on Myshkin, asking him why he finds her attractive. She is fascinated by the prince's confession concerning his meeting with Rogozhin on the train but she is even more fascinated when Rogozhin himself suddenly storms into the Ivolgin house.
Rogozhin is shocked to discover Nastasya, but he is not deterred from his purpose in coming. He offers to buy Nastasya from Ganya; then, he offers to pay a final bid of 100,000 rubles to Nastasya — in exchange for herself. The company is thrown into an uproar. Varya condemns Nastasya, and Ganya, lashing out to slap his sister, strikes Myshkin, who has stepped in front of the girl. This chivalry quiets the party and, after Rogozhin and Nastasya leave, Myshkin is taken to his room.
Later that night, Myshkin goes to Nastasya Filippovna's birthday party. He is uninvited but is welcomed by Nastasya. After champagne is served and games are played, Nastasya announces that, at last, she will reveal whether or not she has decided to marry Ganya Ivolgin. She turns to Myshkin and says that his decision will be her fate. Myshkin whispers that she must not marry Ganya and, amidst loud protestations, Rogozhin and his noisy band of followers are ushered in. Rogozhin has brought the 100,000 rubles he promised to pay Nastasya, but Nastasya changes her mind; she will begin her new life penniless. Under those conditions, no one would have her. Myshkin objects; he would marry her and he reveals that he is heir to a fortune of a million and a half rubles. Nastasya instantly agrees to Myshkin's proposal, then impulsively refuses and flees into the night with Rogozhin.
Myshkin follows the couple, and after the three live for a time in Moscow (where Nastasya decides to marry Rogozhin, then refuses, and comes to Myshkin for comfort), Myshkin returns to Petersburg. He visits Lebedyev (a fellow he met on the train with Rogozhin) and also visits Rogozhin. He tries to convince Rogozhin that the two men are not rivals for Nastasya and, seemingly, Rogozhin understands and swears to be Myshkin's spiritual brother. But after Myshkin leaves Rogozhin, he wanders absently and finds himself at Nastasya Filippovna's house. The girl is not at home but Myshkin senses that Rogozhin has followed him. He is correct; Rogozhin has followed Myshkin to see if he would go to Nastasya as the victorious suitor. Rogozhin then follows Myshkin back to his hotel and attempts to murder the prince but Myshkin suffers an epileptic attack when he realizes that it is Rogozhin who is hiding in the dark with a raised knife. The attack saves the prince's life.
Recuperating at Lebedyev's villa in Pavlovsk Myshkin is visited by the Epanchins and, during their visit, several young men demand to see the prince. Before the Epanchins, the young men demand money from Myshkin; he is not the true son of Pavlishtchev, while Burdovsky (one of the intruders) is, although illegitimate, and they demand that Myshkin give Burdovsky his rightful inheritance.
Myshkin exposes the young men as frauds; he has known of Burdovsky's claim and has had Ganya verifying the facts of the case. Myshkin says that he will not give the men conscience money, but, he says, because Pavlishtchev did take an interest in Burdovsky (the boy is the nephew of a woman Pavlishtchev was once in love with), he offers to give Burdovsky 10,000 rubles — approximately what Pavlishtchev spent on Myshkin's education and medical bills. Madame Epanchin is aghast at such unnecessary extravagance and, after Ippolit, one of the young men, damns the company, she gathers up her family and leaves.
As they leave, a carriage drives up and a voice shouts to Radomsky, an admirer of Aglaia Epanchin, about some lOUs; Myshkin recognizes the voice as Nastasya Filippovna's.
Soon afterward, Myshkin accompanies the Epanchins and friends to the park, and when Nastasya enters, and is insulted, Myshkin rushes to her aid and defends her from the wrath of an army officer. Aglaia is sure that Myshkin will be forced to duel with the officer, but Myshkin is unconcerned. He celebrates his birthday and, afterward, the company listens to Ippolit as he reads a long testament describing his thoughts on death and life, then raises a pocket pistol to his temple and fires. There is no cap in the gun and the company roars in laughter. Ippolit swears that he meant to kill himself, then collapses.
Unable to sleep, Myshkin goes walking in the park, falls asleep on a bench, and is awakened by Aglaia Epanchin. She tells Myshkin that she can no longer bear her family and that she wants to elope with Myshkin to Europe. Myshkin says that the idea is preposterous and, suddenly, Madame Epanchin stands before them demanding an explanation.
After Myshkin confesses that he would like to marry Aglaia, and Aglaia consents, a betrothal party is planned: Myshkin must be introduced to Petersburg society. The party is a disaster. Myshkin launches into one of his long-winded, emotional tirades — this time against the Catholic church — breaks an expensive vase, and finally collapses in an epileptic seizure.
When Myshkin recovers, Aglaia asks him to go with her to Nastasya Filippovna's house and Myshkin agrees. Rogozhin is also there and he and Myshkin witness a violent argument between the two women. Nastasya has hoped to arrange a marriage between Aglaia and Myshkin, but Aglaia accuses Nastasya of toying with the prince and damns her for her past and for her corrupting Myshkin. To retaliate, Nastasya reminds Aglaia that Myshkin has promised to marry her and that she can force him to do so. Myshkin acquiesces and tries to follow Aglaia as she runs from the house, but turns to aid Nastasya when he sees that she has fainted.
The wedding is arranged and Myshkin waits at the church for Nastasya. Nastasya, however, has changed her mind again. She cannot bring herself to marry the prince; he is too good and she too tarnished. She asks Rogozhin to take her away. Myshkin is told that the two have fled and he goes into Petersburg and finally finds Rogozhin waiting for him. Rogozhin takes the prince home with him and shows him Nastasya's body. He has killed her and laid her on a bed. She is covered with good leather and surrounded by bottles of uncorked disinfectant.
Myshkin ministers to the feverish Rogozhin, soothing his brow and cheeks, and when the two are discovered, Rogozhin is babbling and Myshkin has lost his sanity. Eventually Rogozhin recovers from brain fever and is tried and sentenced to Siberia. Myshkin is taken back to Dr. Schneider's sanitarium in Switzerland; the possibilities for his ever recovering are slight.