One thing I love about a Joseph Conrad tale is that, at the outset, you have no idea where it's headed; every tale that I've read is structured differently than every other, and although each story has something to say about the darkness hidden and revealed in human nature, Conrad always comes at this from a different angle.
Marlow, the same man who narrates "Heart of Darkness," narrates Lord Jim. You thought he was voluble in Darkness? He goes on at such length in Lord Jim, Conrad must, in his introduction to the work, address the issue raised by the book's critics of whether someone could believably talk for so long or do so and still possess any listeners. (Conrad suggests—surely as a kind of "up yours" to his critics—that everyone likely stopped at various points for meal breaks.)
Though the novel begins with anonymous narration, it shifts, after introducing us to the somewhat vacuous Jim, to Marlow's account, jumping ahead in time, past some crucial event which Marlow slowly gets around to revealing. (His listeners must know how the event in question played out, at least at the larger level if not personally for Jim; as such, you can't see Marlow as actually withholding information from his true audience. We alone, the readers, are in the dark.) This shipboard crisis is the strongest section of the novel, with Marlow allowing us Jim's own account of his actions (and inactions). Conrad is always great when dealing with suspenseful marine situations, and there's nothing to match it in the rest of the novel, largely because the rest of the action occurs on land. A late event does involve a split in action between land and water, but we're inland, not in open ocean.
The plot, such as it is, involves the fallout from this early crisis of Jim's—where does he go and what does he do to redeem himself? Marlow helps him find work, but Jim's past always pursues him. Then, Kurtz-like, he takes charge in a jungle paradise (the politics of his position are confusing, as there are powers within powers at work) until, in ways that are shatteringly unfair, Jim's past returns, along with fellow whites who possess no honor at all. Jim, a "Romantic" soul, is undone and missteps. Marlow seems to take the position that Jim is wrong in his concluding actions, though, as he repeatedly says, Jim is "one of us."
The novel would have been helped by being tighter. Conrad is terribly repetitive at times, saying something three nearly identical ways when one or two would have done. Additionally, Marlow stops after nearly every moment of speech or action to draw our attention past the action to the seemingly cosmic aspect of human behavior against a chaotic backdrop. It becomes ridiculous quickly, with every minor act leading to overwrought, if often lovely, statements.
Nevertheless, Conrad has much to say here about our sense of fairness and honor, what we think the world owes us and what we might owe the world. Jim remains an unclear figure, a man who never becomes fully formed—unless it's in his final, hopeful gesture.