The Idiot is a novel written by the 19th-century Russian author Fyodor Dostoyevsky. It was first published serially in The Russian Messenger between 1868 and 1869. The Idiot is ranked beside some of Dostoyevsky's other works as one of the most brilliant literary achievements of the "Golden Age" of Russian literature.

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 epilepsy and supposed intellectual deficiencies. On the train journey to Russia, Myshkin meets Parfyon Semyonovich Rogozhin, and is struck by his passionate intensity, particularly in relation to a beautiful woman with whom he is obsessed.

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 last being the youngest and the most beautiful.

General Yepanchin has an ambitious and vain assistant named Gavrila Ardalyonovich Ivolgin (nicknamed Ganya) whom Myshkin also meets during his visit to the household. Ganya, though actually in love with Aglaya, is trying to marry Anastassya Filippovna Barashkov, an extraordinarily beautiful femme fatale who was once the mistress of the aristocrat Totsky. Totsky has promised Ganya 75,000 rubles if he marries the "fallen" Nastassya Filippovna instead. As Myshkin is so innocent and naïve, Ganya openly discusses the subject of the proposed marriage in front of the prince. It turns out that Nastassya Filippovna is the same woman pursued obsessively by Rogozhin, and Ganya asks the Prince whether Rogozhin would marry her. The Prince replies that he might well marry her and then murder her a week later.

The prince rents a room in the Ivolgin apartment, also occupied by Ganya; Ganya's sister Varvara Ardalyonovna (Varya); his mother, Nina Alexandrovna; his teenage brother, Nikolai (Kolya); his father, General Ivolgin; and another lodger named Ferdyshchenko.

Nastassya Filippovna arrives and insults Ganya's family, which has refused to accept her as a possible wife for Ganya. Myshkin restrains her from continuing. The insult is compounded by the arrival of Rogozhin accompanied by a rowdy crowd of drunks and rogues. On the strength of his newly inherited fortune, Rogozhin promises to bring 100,000 rubles to Nastassya Filippovna's birthday party that evening, at which she is to announce whom she shall marry.

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 others. With the acquiescence of Kolya, Prince Myshkin arrives, uninvited. Following Myshkin's advice, Nastassya Filippovna refuses Ganya's proposal. Rogozhin arrives with the promised 100,000 rubles, but Myshkin himself offers to marry Nastassya Filippovna instead, announcing that he has recently received a large inheritance. Though surprised and deeply touched by Myshkin's love, Nastassya Filippovna, after throwing the 100,000 rubles in the fire and telling Ganya they are his if he wants to get them out, chooses to leave with Rogozhin. Myshkin follows them.

For the next six months or so Nastassya Filippovna is torn between Myshkin's compassionate and insightful love for her and a self-punishing desire to ruin herself by submitting to Rogozhin's passion. Myshkin is tormented by her suffering, and Rogozhin is tormented by her love for Myshkin and frequently expressed disdain for his own claims on her. Myshkin's inheritance turns out to be smaller than expected and shrinks further as he satisfies the often fraudulent claims of creditors and alleged relatives. Finally, he returns to St. Petersburg and visits Rogozhin's house. They discuss religion and exchange crosses. But the main topic of their discussion is Nastassya Filippovna. Myshkin becomes increasingly horrified at Rogozhin's attitude to her. Rogozhin confesses to beating her in a jealous rage, and raises the possibility of cutting her throat.

Later that day, Rogozhin, motivated by jealousy, attempts to stab Myshkin in the hall of the prince's hotel, but an unanticipated epileptic fit saves the prince. Myshkin then leaves St. Petersburg for Pavlovsk, a nearby town popular as a summer residence of St. Petersburg nobility. The prince rents several rooms from Lebedev, a rogue functionary who is, however, a highly complex character, first introduced at the time Myshkin meets Rogozhin on the train to Petersburg. 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 to be the son of Myshkin's late benefactor, Pavlishchev, demands money from Myshkin as a "just" reimbursement for Pavlishchev's support. 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 willing to help Burdovsky financially.

The prince now spends much of his time at the Yepanchins'. He falls in love with Aglaya and she appears to reciprocate his feelings. A haughty, willful, and capricious girl, she refuses to publicly admit her love and in fact often openly mocks him. Yet her family begins to acknowledge him as her fiancé and even stages a dinner party in the couple's honor for members of the Russian nobility.

Over the course of an ardent speech on religion and the future of aristocracy, Myshkin accidentally breaks a beautiful Chinese vase. Later that evening he suffers a mild epileptic fit. Guests and family agree that the sickly prince is not a good match for Aglaya.

Yet Aglaya does not renounce Myshkin and even arranges to meet Nastassya Filippovna, who has been writing her letters in an attempt to persuade her to marry Myshkin. At the meeting the two women confront the Prince and demand that he choose between Aglaya, whom he loves romantically, and Nastassya Filippovna, for whom he has compassionate pity. Myshkin demurs, prompting Aglaya to depart, ending all hope for an engagement between them. Nastassya Filippovna then renews her vow to marry the Prince, but goes off with Rogozhin instead.

The prince follows Nastassya and Rogozhin to St. Petersburg and learns that Rogozhin has slain Nastassya Filippovna during the night. The two men keep vigil over her body, which Rogozhin has laid out in his study. Rogozhin is sentenced to fifteen years of hard labor in Siberia, Myshkin goes mad and returns to the sanitorium and Aglaya, against the wishes of her family, marries a wealthy, exiled Polish count who later is discovered to be neither wealthy, nor a count, nor an exile – at least, not a political exile –, and who, along with a Catholic priest, has turned her against her family.