In the present, Marlow starts to tell Jim's story. In that story, Jim starts his career as a sailor, and appears to be pretty darn successful, at least for the time being.
Marlow drops all kinds of hints about trouble brewing in Jim's life at sea. Then we find out Jim is on trial. For what? We'll just have to wait and see. All we know is there is some serious conflict afoot.
Marlow and Jim become fast (if a little surprising) friends. The two start to discuss Jim's affairs as Jim's trial goes on, and we finally learn about the scandalous incident aboard the Patna, but not exactly what went down. We just know it was bad news.
Jim finally tells Marlow the whole story (which Marlow tells us, of course). The lowdown on what really happened aboard the Patna answers many questions that have been brewing in Marlow and us readers, and gives us a clear impression of Jim's character. He's fighting all kinds of inner demons.
Jim takes odd job after odd job. After struggling to find a place where he can live down his bad reputation, Jim lands on Patusan, where he starts fresh and quickly becomes known as Lord Jim. In the present, Marlow stops telling his story on the verandah, but if we assume that's the end of Jim's story, we are sorely mistaken.
The rest of the story comes to us through a series of letters from Marlow. We learn that everything is a-okay for Jim on Patusan (that is until the no-good Gentleman Brown arrives). Jim has a girlfriend, a makeshift family, and hero status. But all is not going to end well.
When Gentleman Brown arrives on Patusan, things go south, and fast. With the help of a few scheming characters, Gentleman Brown attacks Patusan's residents, resulting in all-out war with Jim and his forces. When Gentleman Brown kills Jim's best bud Dain Waris, Jim takes full responsibility, allowing Dain's papa, Doramin, to shoot Jim dead. It doesn't get much more conclusive than that.