The wedding between Prince Myshkin and Nastassya Filippovna is set for a week after Radomsky's visit to the prince. General Ivolgin dies of a second stroke eight days after his first. At the general's funeral Myshkin thinks he sees Rogozhin. Concurrently with the wedding preparations, Myshkin spends a great deal of time with the Ivolgins as well as with Hippolite, who warns the prince about Rogozhin. Meanwhile, Lebedev schemes to have the prince confined to a mental asylum; Lebedev even summons a doctor, but the physician thinks Myshkin is in very good mental health.
Several days before the wedding, the prince finds Nastassya Filippovna in a terribly distressed state, so he stays with her to calm her down. The day of the wedding, however, she looks exquisite. A large crowd gathers in the church and near her house and Lebedev's house. As Nastassya Filippovna descends the stairs, she stares into the crowd and sees Rogozhin. Suddenly she runs to him and asks him to take her away. They quickly go to the train station and leave for St. Petersburg.
The next morning, Myshkin catches the first train to St. Petersburg. He goes straight to Rogozhin's house but there he is told that the man is not home. The prince then goes to the house of a friend of Nastassya Filippovna's where she frequently stays while in St. Petersburg. Myshkin is told that she is not there, so he returns to Rogozhin's but once again is told that neither she nor Rogozhin are there. The prince goes back and forth among Rogozhin's house and several houses where Nastassya Filippovna has friends in St. Petersburg, but has no luck.
Finally, in the evening, Myshkin goes to the same hotel where he stayed last time and where he had his fit. He thinks that if Rogozhin needs him, he will think of finding him there. The prince is not mistaken. In the evening, just outside the hotel he meets Rogozhin, who invites him to follow him, although on opposite sides of the street. They go to Rogozhin's house and enter the dark rooms; all the windows are covered. In the study Myshkin sees Nastassya Filippovna, who is lying on the sofa, covered by a sheet; Rogozhin stabbed her the previous night. Rogozhin invites the prince to lie down on the floor next to her. Later, when the doors open and people enter, they find Rogozhin in a delirious state and Myshkin gently stroking him.
Rogozhin is tried and sentenced to fifteen years of hard labor in Siberia. Hippolite dies two weeks after Nastassya Filippovna. Myshkin is sent to Dr. Schneider's clinic in Switzerland, where he returns to the same unwell state he was in when he was first brought to the clinic several years earlier. The Yepanchins and Radomsky occasionally visit Myshkin, but he does not recognize them very well. Aglaya Yepanchin runs off with an alleged Polish count who turns out to have lied both about his nobility and his fortune. Radomsky keeps in touch with the Yepanchins and writes to them of his visits to the prince. Radomsky also keeps in touch with Kolya and Vera Lebedev, and there is even a hint of the beginning of a romantic relationship between him and Vera.
Nastassya Filippovna tries to save herself for the last time by marrying Myshkin, but again she ultimately proves unable to go through with it. Her self-blame overpowers her self-love and her desire to marry the Prince, though she does truly love him. At the last minute, immediately before the wedding, she runs to Rogozhin, most likely aware that she is running to her death. Rogozhin tells Myshkin that Nastassya Filippovna even insists on going to his house when they reach St. Petersburg—another indication that her escape with Rogozhin is a sort of suicide on her part. She chooses to die, as she sees her only two choices in life as either living with her shame or ruining the prince's life by marrying him. Although Myshkin, in his desire to help her, is perfectly willing to marry her, she realizes that he still loves Aglaya. Thus, the prince is unable to save Nastassya Filippovna from death, much as he is unable to save General Ivolgin, who dies shortly after the first stroke. In parallel with Nastassya Filippovna, Rogozhin too is ruined. Having killed her, he is not only sentenced to a labor camp in Siberia, but also descends further into a state of mental illness. Myshkin is unable to do anything to prevent the ruin of either Nastassya Filippovna or Rogozhin. All he can do is stroke their heads like children in an attempt to comfort them.
After the meeting between Aglaya and Nastassya Filippovna, Myshkin begins to lose the last remnants of mental stability, which vanish entirely after Nastassya Filippovna's murder. Mirroring the decline in the prince's sanity, the narrator gradually loses omniscience throughout the novel. In the end, the narrator resorts to rumors and gossip in constructing the novel's events and cannot tell with any degree of certainty whether the events of the wedding actually occurred in the way he is about to tell. Much like Myshkin, who becomes more and more of an "idiot" as the novel progresses, the narrator too approaches idiocy in his inability to retell the novel's events with any certainty or interpret them with any credible explanation.
The Idiot ends having gone full circle geographically. Myshkin takes a train from Switzerland to St. Petersburg, follows Nastassya Filippovna to other cities, goes to Pavlovsk, returns to St. Petersburg, and finally is sent back to the clinic in Switzerland. The prince arrives in St. Petersburg like a messiah sent to help many of the main characters of the novel; however, he fails to save them from destruction and then returns to the clinic in a state of complete mental degradation. It may be extreme, however, to characterize his trip as a complete failure. Certainly, at the end of the novel, Nastassya Filippovna and General Ivolgin are dead, while the lives of Rogozhin and Aglaya are virtually ruined. Nonetheless, Myshkin has had an indelible effect upon the people he has met, particularly younger characters like Kolya and Vera Lebedev. Perhaps it is Dostoevsky's hope that Myshkin will also have an effect on us as readers. For now, however, the author suggests that society is not ready for a man of such profound goodness as the prince.