Prince Lev Nikolayevich Myshkin, a fair-haired young man in his late twenties and a descendant of one of the oldest Russian lines of nobility, arrives in St. Petersburg on a November morning. He has spent the last four years in a Swiss clinic for treatment of his "idiocy" and epilepsy.
Myshkin's only relation in St. Petersburg is the very distant Lizaveta Prokofyevna Yepanchin. Madame Yepanchin is the wife of General Yepanchin, a wealthy and respected man in his late fifties. The prince makes the acquaintance of the Yepanchins, who have three daughters, Alexandra, Adelaida, and Aglaya, the latter being the youngest and the most beautiful.
General Yepanchin has an ambitious and rather vain assistant by the name of Gavril Ardalyonovich Ivolgin (nicknamed Ganya) whom Myshkin also meets during his visit to the household. Ganya, though he is actually in love with Aglaya, is in the midst of trying to marry Anastassya Filippovna Barashkov, an extraordinarily beautiful "fatal woman" who was once the mistress of the aristocrat Totsky. Totsky has promised Ganya 75,000 rubles if he marries the "fallen" Nastassya Filippovna. As Myshkin is so innocent and naïve, Ganya openly discusses the subject of the proposed marriage in front of the prince.
The prince rents a room in the Ivolgin apartment, also occupied by Ganya, his sister, Varvara Ardalyonovna (Varya); his mother, Nina Alexandrovna; teenage brother, Nikolai (Kolya); his father, General Ivolgin; and another lodger by the name of Ferdyshchenko.
Nastassya Filippovna arrives and attempts to insult Ganya's family, which has refused to accept her as a possible wife for Ganya. Myshkin, however, stops her, putting her behavior to shame. Suddenly a rowdy crowd of drunks and rogues arrives, headed by Parfyon Semyonovich Rogozhin, a dark-haired twenty-seven-year-old who is passionately in love with Nastassya Filippovna. Rogozhin promises to bring 100,000 rubles to Nastassya Filippovna's birthday party scheduled for that evening at which she is to announce whether she will marry Ganya or not.
Among the guests at the party are Totsky, General Yepanchin, Ganya, Ferdyshchenko, Ptitsyn—a usurer friend of Ganya's who is a suitor to Varya Ivolgin—and a few others. With the help of Kolya, Prince Myshkin arrives as well, though uninvited. Following the prince's advice, Nastassya Filippovna refuses Ganya's proposal. Rogozhin arrives with the promised 100,000 rubles, but suddenly Myshkin himself offers to marry Nastassya Filippovna, announcing that he has recently learned he has a large inheritance. Though shocked at such a generous offer by an honest and generous heart, Nastassya Filippovna only deems herself worthy of being with Rogozhin, so she leaves the party with Rogozhin and his gang.
Prince Myshkin spends the next six months following Nastassya Filippovna as she runs from Rogozhin to the prince and back. Myshkin's inheritance turns out to be smaller than expected, and it shrinks further as he satisfies the claims of creditors and alleged relatives, many of which are fraudulent. Finally, the Prince returns to St. Petersburg and visits Rogozhin's house, which is a dark and dreary place. They discuss religion and exchange crosses.
However, later that day, Rogozhin attempts to stab Myshkin in the hall of the prince's hotel, but the prince is saved when he has a sudden epileptic fit. Several days later, Myshkin leaves for Pavlovsk, a nearby town popular for summer residence among St. Petersburg nobility. The prince rents several rooms from Lebedev, a rogue functionary. Most of the novel's characters—the Yepanchins, the Ivolgins, Varya and her husband Ptitsyn, and Nastassya Filippovna—spend the summer in Pavlovsk as well.
Burdovsky, a young man who claims himself to be the son of Myshkin's late benefactor, Pavlishchev, comes to the prince and demands money from him as a "just" reimbursement for Pavlishchev's support of the Prince. Burdovsky is supported by a group of insolent young men who include the consumptive seventeen-year old Hippolite Terentyev, a friend of Kolya Ivolgin. Although Burdovsky's claim is obviously fraudulent—he is not Pavlishchev's son at all—Myshkin is ready and willing to help Burdovsky financially.
The prince spends much of his time at the Yepanchins'. Soon, those around him realize that he is in love with Aglaya and that she most likely returns his feelings. However, a haughty, willful, and capricious girl, she refuses to admit her love for Prince Myshkin, and often even openly mocks him. Aglaya's family begins to treat him as her fiancé, and they even hold a dinner party with many renowned guests who are members of Russian high society.
Myshkin, during the course of an agitated and ardent speech on religion and the future of aristocracy, accidentally breaks a beautiful Chinese vase. Later in the evening he has a mild epileptic fit. The guests and the family are convinced that the seemingly sickly prince is not a good match for Aglaya.
Aglaya, however, does not renounce Myshkin, and even arranges a meeting between herself and Nastassya Filippovna, who has been writing letters to Aglaya to convince her to marry Myshkin. During this meeting, the two women force the Prince to choose between his romantic love for Aglaya and his compassionate pity-love for Nastassya Filippovna. Myshkin hesitates briefly, which prompts Aglaya to run off, breaking all hope of an engagement between them. Nastassya Filippovna wishes to marry the Prince again, but in the end she proves unable to bring herself to do so, instead running off with Rogozhin at the last minute.
The Prince follows the two to St. Petersburg the next day and finds that Rogozhin has stabbed Nastassya Filippovna during the night. The two men keep vigil over her body, which Rogozhin has laid out in his study. The epilogue relates that Rogozhin is sentenced to fifteen years of hard labor in Siberia, that Prince Myshkin loses his mind and returns to the Swiss sanitarium, and that Aglaya leaves Russia with a Polish count who lies to her and soon abandons her.