Spoilers ahead so do not read if you are planning to see this dumpster fire of a movie one day:

Plot and Ending A: this movie is your usual action movie in a sci-fi setting. Will Smith is all alone in a post apocalyptic world and he’s hiding from humanoid monsters. Eventually he captures one and studies it to learn about them. The monsters follow him to his house/lab and he kills them all like the hero he is.

Ending B: this is closer to the real book ending (or so they say I never read it). In the end we learn that the reason these monsters were following Will Smith’s character was because the one he captured earlier is actually the daughter of one of them and that they are actually not savage beings but closer to humans in thought and feelings. The real monster all along was Will Smith’s character... he was killing them and chasing them and kidnapping them. They were afraid of him and not the other way around.