The Grand Inquisitor's monologue at the heart of this long novel is one of the greatest passages in literature, but it is Christ's response that chills and haunts us and makes us weep. Dostoyevsky (being Russian, and therefore dark and brooding) was apparently incapable of short fiction, but in this case (his last novel) it's all worth it. The Brothers Karamazov is philosophical, thoughtful and brilliant, of course, but it is also a fascinating narrative depicting the simultaneous moral rise and fall of various members of the Karamazov family, particularly the four brothers and their degenerate father.
Philosophical asides and discursives don't fetter this novel—they are its soul and substance. If you don't like intensely reflective fiction you probably won't like this book ("probably" here meaning "most certainly"), but if you want to be well-read you will read Dostoevsky's masterwork. The author's subject is human nature, and his anguished perceptions are as relevant now as they ever have been. Some of the content is disturbing (old Fyodor, having sunk to degeneration himself at times, doesn't flinch from depictions of sin and evil), but the ultimate vision is both realistic and Christian in a way that few works beside the Bible have managed.
The main character is Alyosha, the youngest Karamazov and a novitiate monk in the local monastery. He is sharply contrasted to his atheist brother Ivan, and in many ways represents Christ whereas Ivan represents mankind. The Grand Inquisitor/Christ exchange comes in the middle of a debate between Alyosha and Ivan concerning the nature of God and man, and is actually a parable told by Ivan to illustrate man's inherent selfishness and inability to follow the Christian God, should such an entity exist. The answer to Ivan's nihilism is not another parable or monologue, but specifically the subsequent description of Father Zosima's goodness and more broadly Alyosha's selflessness and purity throughout. Given the choice, Ivan posits, man will always choose the security of slavery over the risk of freedom—Alyosha counters simply by choosing goodness despite the suffering it exacts.
In many ways The Brothers Karamazov ends in despair. The dissolution of the family is as much the result of misdirected goodness as of depravity, and while many questions about faith and spirituality are raised, few (if any) are answered. This, of course, is simply Dostoevsky's intensely honest representation of human existence—always uncertain and certainly painful. The conclusion is not wholly tragic, however: one good deed of Alyosha's remains amid the darkness, evidence that goodness will never capitulate wholly to the madness of evil, evidence ultimately that God's hand, though often obscured by human wickedness, still moves among creation keeping hope alive.