At the Epanchin villa, Myshkin explains to Lizaveta Prokofyevna that Aglaia arranged their early morning meeting and that they spoke of matters that concerned only Aglaia. He is allowed to leave. He goes back to his villa but is unable to go to bed, for first he is bothered by Vera Lukyanovna's chatter, then by Kolya, and finally by Lebedyev. Very solemnly, Lebedyev says that he has lost four hundred rubles; how, he's not quite sure. He suspects robbery, however, and he thinks that Ferdyshtchenko might be the culprit. Ferdyshtchenko (purposely, Lebedyev feels) announced early in the morning that he was leaving to go to a specific address. And, say's Lebedyev, the address has proved to be false. Also Lebedyev thinks it curious that General Ivolgin insists on accompanying him to Petersburg in search of Ferdyshtchenko. He tells Myshkin that he plans to follow the general (secretly, of course) to the widow's houuse. There, he plans to surprise the general and shame him in the name of his family. The widow has forbidden the general to come see her unless he brings money with him. For the general's sake, Lebedyev thinks that Myshkin should be present at the widow's house.
Myshkin cautions Lebedyev to be especially careful before he presses unfounded charges against Ferdyshtchenko; Kolya has said that one must be very careful around him. Then, on one condition, the prince agrees to help Lebedyev: His landlord must make no uproar, and nobody must know. Lebedyev swears.
It is evening before Myshkin reads the letters that Aglaia gave to him, letters Nastasya has written to Aglaia. They are nightmarish, with Nastasya humbling herself and assuring Aglaia that the two women are opposites and that she is infinitely below Aglaia. For Nastasya, Aglaia is perfection; in fact, Aglaia and Myshkin are one. Aglaia "can love without egoism" — she is innocent. Nastasya says further that she wants to bring Aglaia and Myshkin together for one reason — selfishly, for her own sake. Nastasya says that she herself has almost ceased to exist. She feels the weight of Rogozhin's eyes and is sure that a secret lies in his gloomy house: A razor wrapped in silk is waiting for her. Rogozhin loves her but will kill her.
Walking in the darkness, pondering Nastasya's letters, Myshkin is startled to see Nastasya appear before him. She sinks to her knees, asking him whether or not he is happy and whether he has been with Aglaia. She says that she is going away, for the last time, and clutches at Myshkin's hands before dashing away. Rogozhin appears then, beside Nastasya, and cries to Myshkin that he will return; that Myshkin must wait for him. Within five minutes Rogozhin is back and tells the prince that Nastasya has promised to write no more letters to Aglaia and that she is leaving, as Myshkin wishes. Rogozhin adds that he too has read Nastasya's letters, even the one about the razor; Nastasya showed him each of the letters, he laughs. He then turns to Myshkin and tells him to remember no evil against him, and asks him one last question before going — the same question Nastasya asked the prince: Are you happy or not? Sadly, Myshkin answers that he is not.
Myshkin's scene with Lebedyev concerning the theft of the pocketbook is a comic interlude before the last scene with Nastasya Filippovna and Rogozhin. Lebedyev is obsessed with games and intrigues and secrets, and Dostoevsky is showing us this again, plus reminding us of Myshkin's naiveté. By degrees, we realize that Myshkin's willingness to help other people and to believe what they say — even Lebedyev — can be wrong.
Nastasya's letters to Aglaia do not sound as mad as Myshkin believes her to be. Instead, they are pathetic because Nastasya believes herself to be so degraded that she has made an idol of Aglaia, and, because she feels compassion for Myshkin, Nastasya hopes to unite Aglaia and Myshkin, a union of perfect souls. This will be her contribution before she consummates (by death, she fears) an antithetical union between herself and Rogozhin. She is so miserably unhappy, and she wants Myshkin to be happy; it is right that he should be but, as the prince tells Rogozhin, he is unable to be happy.
As Part III ends, Myshkin is no doubt sure that Rogozhin will murder Nastasya and that Aglaia is about to do something rash, out of spite. The women he cares most deeply for are destined for tragedy. Is the cause? His conscience demands that he answer affirmatively.