Prince Lev Nikolayevich Myshkin comes to Petersburg by train from Switzerland, where he's been a long-term patient for debilitating epilepsy. On the train, he meets Parfyon Rogozhin and Lebedev.
The only person he knows to contact in Russia is a very distant relative—Lizaveta Epanchin, the wife of General Epanchin, and mom to three daughters. They are a part of the social elite, but for some reason they are slightly odd in their social circle. When Myshkin shows up, he kind of just charms/disarms his way into the family, with General Epanchin giving him a job, Mrs. Epanchin taking him under her wing, and Aglaya, the youngest and most beautiful daughter, seeming to take an interest in him. The feeling is mutual.
Myshkin shows up just as a lot of crap is about to hit the fan. General Epanchin has a gross old man friend, Totsky, who a few years ago had kept a mistress, Nastasya Flippovna Barashkov. Because that whole arrangement was pretty skeezy, she is now out for revenge and so Totsky wants help marrying her off in order to stop being harassed. General Epanchin's ambitious and vain employee Gavril Ivolgin (Ganya) is actually in love with Aglaya, but can't pass up the opportunity to make bank, and so has agreed to marry Nastasya for seventy-five thousand roubles. Which, ewww.
Anyway, Myshkin shows up on the day of Nastasya's birthday party, when she is supposed to tell everyone if she'll marry Ganya or not—since the prince seems like a simpleton, everyone just talks about this in front of him so he's got all the info. He gets a load of Nastasya's portrait and is totally struck by her beauty and the deep suffering he reads on her face.
Myshkin agrees to rent a room at the Ivolgins' house, and there he meets Kolya, who is Ganya's idealistic teenage brother, Varya, who is Ganya's conniving but way more practical sister, and General Ivolgin, who is Ganya's totally alcoholic, delusional, and generally horribly embarrassing dad.
Nastasya shows up and tells off Ganya's family, who are all pretty grossed out by the idea that this "fallen woman" would be one of them through marriage. Then Rogozhin, who is in crazy love/lust with her, busts in with a posse of drunks and says he'll bring a hundred thousand to the birthday party that night to show up Ganya and his seventy-five.
That night, Myshkin party-crashes the birthday. He's pretty struck when he actually meets Nastasya and she seems to also recognize him as something special. She asks him whether she should marry Ganya and he says no, so she refuses Ganya. Rogozhin shows up with his drunken buffoons and really does have that hundred grand he promised. But Myshkin then also proposes to Nastasya, suddenly busting out a letter that shows that he has a sizable inheritance. Nastasya leaves with Rogozhin, but is clearly at least kind of into Myshkin's offer. Oh, and also she offers that hundred grand to Ganya by throwing it into the fireplace and telling him to go get it. He wants to, but doesn't.
For the next six months, Nastasya goes back and forth between Myshkin and Rogozhin, running from one to the other. Myshkin is really soft-hearted and totally innocent, so his fortune is greatly diminished by various con artists who hit him up for money. Finally, she runs away from both of them, and the prince comes back to St. Petersburg and goes to hang with his old frenemy Rogozhin. They discuss religion and exchange crosses.
That same day, a totally jealous Rogozhin tries to stab Myshkin, but the prince is saved by a sudden epileptic seizure. Myshkin takes off for Pavlovsk, a vacation town near Petersburg where the Epanchins and the Ivolgins, and, as it turns out, Nastasya, are already staying. Myshkin rents rooms from Lebedev, the dude he met on the train, who is some kind of government flunkey.
Almost immediately, they get a weird visit from a bunch of rude nihilists. One of them, Burdovsky, claims to be the son of Myshkin's dead benefactor Pavlichev, and he is there to extort some money. The rest of his crew includes Ippolit Terentyev, a teenager dying of tuberculosis, and Keller, an ex-military tough guy. After they present their case, Myshkin whips out his secret weapon—Ganya, who has totally done his homework and quickly debunks the whole Burdovsky claim. Myshkin still says he wants to help Burdovsky financially, and also decides to put up the dying Ippolit in his house. Dude.
Myshkin now spends most of his time with the Epanchins. He's awkward and pretty weird, but he falls in love with Aglaya and she seems really into him also. But, she's moody and has that hot-girl-who-knows-she's-hot thing going on for her, so she mostly makes fun of the prince and treats him like a joke to his face. It's clear that part of the issue is that she is way jealous of his relationship with Nastasya. Meanwhile, one night, at Myshkin's birthday party, Ippolit decides to read a really, really long confession letter that is basically a very detailed suicide note. Then he tries to kill himself, but can't because he failed to load the gun. Cue: sad trombone.
In any case, the Epanchins are slowly getting used to the idea of Myshkin being Aglaya's fiancé and decide to throw a big dinner party to show him off to high society. Hopefully he'll get a passing grade and the wedding can go forward. At first, Myshkin shows some self-control at the party, but he is pretty quickly overcome by how charming everyone is—mostly because he doesn't read it as surface charm, but as true displays of feeling. He starts making a wildly inappropriate and way over-excited speech about the aristocracy (since he's one of them), then breaks a vase that Aglaya warned him to watch out for, and then has an epileptic seizure. The guests are kind of stunned by all of this, but still, it looks like he's going to just squeak by with a very low passing grade.
Aglaya is closer than ever to admitting her feelings for Myshkin, but reveals that she has been receiving letters from Nastasya trying to convince her to marry Myshkin. Aglaya arranges to meet Nastasya, and Myshkin comes with her. The meeting does not go smoothly. Shocker. It comes down to the prince having to choose between them, and he can't really figure out whether to go with his romantic love for Aglaya or his compassionate love for Nastasya. Nastasya collapses as Aglaya runs off, and the prince rushes to Nastasya's side. Aglaya considers this the last straw.
Nastasya says she will marry the Prince. As wedding arrangements go forward, she has lots of fits of rage and despair, and he keeps trying to go see Aglaya. That bodes really well. No one wants the wedding to go forward, but still the day comes. Just as Nastasya is about to get into the carriage to go to the church, she runs off with Rogozhin, who takes her back to Petersburg.
Myshkin follows them the next day, and then conducts a frantic day-long search for them. He is found by Rogozhin, who takes him back to his house. There, Myshkin finds the corpse of Nastasya, who has been murdered by Rogozhin the previous night. The two men spend the night near the body, having all sorts of crazy conversations.
The next morning, the police come, and find Rogozhin with a brain fever, and Myshkin back in the near-catatonic state he originally suffered from. Rogozhin is sentenced to fifteen years of hard labor in Siberia, Myshkin returns to the sanatorium with Kolya's help, and Aglaya, against the wishes of her family, marries a wealthy, exiled Polish count that later is discovered to be neither wealthy, nor a count, nor an exile—at least, not a political exile. The Epanchins come to visit Myshkin, and Mrs. Epanchin weeps over him.