When the novel opens, Nastasya is supposed to announce at her birthday party whether she'll marry Ganya or not.
The reason this is a thing? Well, as a young girl, she was raised to be a mistress for a gross old aristocrat named Totsky. Ever since then, she has threatened him with revenge, so he wants to marry her off to be rid of her, and has offered Ganya 75 grand to do it.
That day, Nastasya shows up at Ganya's house and tells off his family for thinking that if he marries her he'll be marrying way down.
After Rogozhin shows up there and offers to bring money to her birthday to buy Ganya out, as it were, she makes him raise his bid for her to a hundred thousand.
At her birthday party, Nastasya meets Myshkin and is immediately struck by his simplicity and honesty. She asks him whether she should marry Ganya, and does as he says (which is not marry him).
When Rogozhin and his crew show up with the money, Nastasya makes a big show of seriously considering Myshkin's proposal, but ends up leaving with Rogozhin.
Just before leaving, she throws Rogozhin's money into the fireplace, and says that Ganya can have it provided he'll stick his hand into the fire to get it.
Nastasya spends six months going back and forth from Rogozhin to Myshkin, trying to decide between being with the violent, passionate one, or the one who loves her out of pity and doesn't have any desire for her.
Finally, she flees both of them and makes for Pavlovsk where she lives a pretty flamboyant life in defiance of everyone being so super scandalized by her and the fact that she had sex outside of marriage.
At the same time, she seems to slowly be going crazy and promises Rogozhin that she will marry him as soon as Myshkin and Aglaya get married.
Nastasya starts sending long, inappropriate letters to Aglaya, trying to convince her to marry Myshkin. Sadly these letters are partly what makes this whole thing backfire.
Aglaya and Myshkin come to see her before their engagement is announced officially because Aglaya is pretty jealous.
At this meeting, Nastasya goes a little nuts, and then commands Myshkin to stick to his earlier promise to marry her, playing on his sense of duty and honor. Then she faints and he rushes to her side.
She and Myshkin start planning for their wedding. Some of the time she seems normal, but every now and again she throws a huge fit of despair and self-doubt, wondering how she could be doing this to him.
On the day of the wedding, just before getting into the carriage to go to the church, she sees Rogozhin in the crowd and runs away with him.
She happily goes to Petersburg with him. There, she agrees to sneak into his house even though she has frequently had premonitions about his violent tendencies. She seems pretty willing to go to her doom.
That night Rogozhin stabs her in the heart and kills her.