So, yeah, lots and lots of conveniently unknown distant relatives had to die in extremely coincidental ways for Myshkin to be next of kin—but now his clichéd inheritance is secure.
Nastasya seems to accept Myshkin's offer of marriage. She calls herself a princess, orders champagne all around, and generally acts superior.
She keeps questioning Myshkin's resolve though. Will he really never feel bad that she's been Totsky's mistress? Will he really be ok with the rest of society thinking of her as used goods? Is he really never going to blame her when he realizes he's married down? (Myshkin answers all of these questions by saying that he will always respect and honor her.)
Under all of this are some super-duper heavy duty self-esteem issues. She feels gross and damaged by Totsky's treatment, but thinks that she is at fault—she should have killed herself or something at least. It's pretty clear that she is basically unable to deal with herself or with anyone who wants to treat her well. We have to say, that is some very modern psychology at work here—for a novel of its time, this is a pretty sophisticated and complex treatment of this woman's emotions. Well done, sir.
Nastasya finally says that she won't marry Myshkin after all. She doesn't want to ruin him, and really he should be with Aglaya Epanchin—remember her? Yeah, us neither. Instead, she wants to go off with Rogozhin to go be a prostitute.
But first, a little insanity.
She tells Ganya that since he loves money so much, the 100 grand that Rogozhin brought (which is now hers for "one night with Rogozhin") is his. Except? He has to dive into the fireplace and snatch it out from the fire with his bare hands.
Ganya looks, waits, thinks about it, but finally walks away from the burning money. Then he faints.
Others beg to retrieve the money, but finally Nastasya just pulls it out herself with some fireplace tongs. She puts the mostly ok money near Ganya—it's his for when he recovers.
Nastasya and Rogozhin and the rest of that crew go off in the troika he has waiting downstairs (basically a carriage with three harnessed horses).
Myshkin runs off after them, despite Epanchin trying to beg him not to.
Epanchin, Ptitsyn, and Totsky briefly consider whether Nastasya is mentally ill. They decide no. And then Totsky says another really gross thing—basically that anyone who met Nastasya could see how he's not really at fault for being so hot for her that he would keep her as a kind of sex slave. Um. Yeah, we're pretty sure that's not a legit defense, dirty old man.