For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Prince Myshkin comes to Petersburg and stumbles into Rogozhin, and the Ivolgin and Epanchin families. They are all trying to figure out what to do with Nastasya Philipovna, a beautiful woman without a proper place.
Act II
Myshkin's innocence and sense of duty makes him propose to Nastasya, thus making enemies of other potential suitors, and complicating his budding relationship with Aglaya. He abandons everything to try to help Nastasya in whatever way he can, eventually asking her to marry him.
Act III
The wedding is interrupted when Nastasya runs off with Rogozhin, who then takes her back to his house in Petersburg and kills her. Myshkin finds him and the corpse, and after a tense and horrible night, becomes yet again almost catatonic.