We'll just level with you here: the narrative technique of Lord Jim is confusing to say the least.
First, we have Marlow, who is the main narrator of the novel. But as he tells Jim's story, other voices creep into the mix as the characters he meets share what they know of Jim. It's as if Marlow is channeling a story with multiple voices into one narrative stream. Plus, there's the fact Marlow is not actually the narrator of the novel at all.
Yep, that's right. There's a whole other, unidentified person who is sitting on the verandah listening to Marlow, and interrupting every once in a while to remind us that Marlow, too, is a character:
Marlow paused to put new life into his expiring cheroot, seemed to forget all about the story, and abruptly began again. (8.10)
Weird, right? Plus, there's the anonymous narrator of the first five chapters, which document Jim's early life. If your head is already spinning, don't worry. Shmoop has your back.
For the sake of sheer practicality, we're going to go ahead and call Lord Jim a first person narrative, because the bulk of the novel is told in Marlow's words. As Conrad's go-to narrator (Marlow also narrated Conrad's first novel, Chance, and his most famous novel, Heart of Darkness), Marlow has his work cut out for him. Lord Jim has a great many stories woven together, and we need someone to tell them to us. That gargantuan task falls to Marlow.
After the first four anonymously narrated chapters, we meet our storyteller at the end of Chapter Four. Every chapter after that uses quotation marks around the paragraphs to indicate that Marlow is speaking. For much of the novel, it's a pretty straightforward narrative; Marlow tells us Jim's story, and how he came to find out about it (through his many, many sources, far and wide).
The only wrench that ever gets thrown is that pesky third person we've already mentioned. Why not have Marlow just narrate the whole darn story?
Part of the reason might be thematic – Lord Jim is largely about storytelling, and Conrad uses multiple storytellers throughout the narrative who all interpret one another and repeat one another. The novel shows us how stories can get filtered and distorted through different people's perspectives, including Marlow's.
Also, the outside narrator means that Marlow functions both as a narrator and an independent character. Bonus, right? Instead of seeing the whole world of Lord Jim through Marlow's eyes, we get one layer of removal that gives us a good dose of perspective. Every time that other narrator rears his anonymous head, we're reminded to take Marlow's words with a grain of salt, because he's only human.
Marlow's voice glides in and out of the story, and we get frequent instances where Marlow slips over to quoting Jim, or Stein, or the French Lieutenant, or, well, you get the picture:
[W]hile his brain and his heart together were pierced as with daggers by panic-stricken screams, "Let go! For God's sake, let go! Let go! She's going." Following upon that the boat-falls ripped through the blocks, and a lot of men began to talk in startled tones under the awnings. "When these beggars did break out, their yelps were enough to wake the dead," he said. (9.21)
Marlow is both paraphrasing Jim and quoting his young protege. How could he possibly know that Jim's brain and heart were pierced with dagger-like screams? Either Marlow is projecting feelings onto Jim, or Jim has described his experience this way, and Marlow is merely restating what he said (perhaps with a little color added). This back-and-forth makes it difficult to suss out who is really saying what, and, more important, who is feeling what.
As it turns out, another reason for not having Marlow be a first-person narrator may be Conrad's interest in the way, by telling a story, we use and reinterpret other peoples voices. Jim's story is complicated, and Marlow has to wrangle it out of various people, put it into his own words, then share it with an audience who may go on to retell it in their own way.
The narrative technique puts us in the same position as Marlow's audience in the story, trying to sort out what he and others are saying, wondering what's true. We're on shaky ground here, and we can't help but think that's exactly where Conrad wants us.