Confused? We don't blame you. Frankenstein is made up of:
Walton's letters, which include
Victor's story, which includes
the monster's story, which includes
Felix's story (told in third person)
What the point of having all these different stories? Here's one idea:
Science is all about trying to figure out how the world works. Victor himself tells us that when he says he wants to "penetrate the secrets of nature" (2.7). So, you could say that the scientist is trying to adopt the perspective of God, who's the ultimate third-person omniscient narrator: he knows everything about everyone.
But in Frankenstein, we never get that perspective. All we get are a bunch of people's stories, and we have to piece together the truth from that. Do we believe the monster's own story about himself? Or do we believe what Victor says about him: that he's a heartless, cruel, ugly monster? You could say that Shelley makes us like scientists, having to piece together imperfect information—and you could also say that, by giving us so many different stories, she shows us it's impossible to ever really know.