The Karamazov brothers return to their father's home, only to discover that their father is an evil, selfish, horny old man.
Fyodor Karamazov is more than just a really, really bad father in Dostoevsky's novel; he's a force of evil, ruining the lives of everyone in his family. All of the Karamazov brothers have to deal with the fact that they may have inherited his evil nature, including the angelic Alyosha.
As time goes on, relations between the brothers and their father worsen, and Alyosha experiences a spiritual crisis when his elder dies.
Each of the brothers tries to come to terms with their father in a different way: Dmitri through violence, Ivan through arrogance and intellect, and Alyosha through purity of heart. But they're no match for Fyodor, and even Alyosha's faith is shaken by the death of the elder Zosima, his spiritual guide and, in some sense, replacement father.
Things come to a head when Fyodor Karamazov is murdered.
It seems that the Karamazovs succumb to the "Karamazov force," the dark and evil nature they seem to have inherited from their father. (Seemingly, because we're dealing with a "Rebirth" plot; skip ahead to Stage E for a spoiler.) Dmitri is the prime suspect in his father's murder, but all the Karamazov brothers must grapple with the fact that they may have been in some way complicit in his death.
Dmitri is falsely accused and convicted; Ivan gets seriously ill; Smerdyakov kills himself; and Alyosha leaves the monastery.
So the novel is a real downer at this point. Dmitri is innocent but is found guilty and awaits exile to Siberia. Ivan gets some kind of brain fever and is reduced to a babbling idiot. Smerdyakov hangs himself. Alyosha abandons the life of a monk. At this point, we have to wonder if there's any point in living in such a terribly unfair world filled with suffering and disappointment.
The novel ends on a note of redemption, as Dmitri looks forward to his exile, Ivan is cared for by the woman he loves, and Alyosha is inspired by the goodness of children.
But wait – things aren't so terrible! Dmitri actually looks forward to his sentence, sort of, because it compels him to mend his ways. Ivan's illness finally wins him Katerina's undivided attention. And Alyosha is revived by the innocent love of Ilyusha's friends.