It is about one man fighting with himself , while it is portrayed as two people (Anthony/Daniel and Adam) only one of them is real, and its Adam, I know this because in the beginning on the phone his mother is speaking and says "Adam honey" and also later on in the movie his mother also tells Adam (who is not an actor) to give up his dreams of being a third rate actor. now the story of this movie is hard to understand , its about Adam and Helens marriage and the remaining spark of an old affair, in the beginning you see it open with Jake Gyllenhaal in what is presumably a strip club, eventually you see a woman come out and step on a spider, the woman is Mary, Adam's affair , after that it opens to Adam sitting in his car as you hear his mother leave a voice mail for him. Throughout the movie you see spiders, spiders represent his affair with Mary, and how he still thinks about it, and when you see Adam with Mary he really isn't its just representing his still lasting feelings for her. you see when Adam calls Anthony it really wasn't Adam, it never happened, the whole phone call scene was his imagination, helping him realize he needs to forget about Mary. I'm gonna skip a bunch of unimportant things and try to go as chronologically as I can. At the point where Anthony tells Adam hes going to have sex with his girlfriend , that's really just Adam thinking to himself about him needing to get rid of Anthony and the memories of Mary. so when Anthony goes and takes Mary out that doesn't really happen. whats happening is Adam is thinking that up to help in the getting rid of memories, it is also revealed that Adam is the real person when Adam and Helen lay down and Helen asks how was school . the scene of Mary and Anthony having sex and Mary freaking out is Adam realizing he is married and needs to get rid of the memories, when Anthony and Mary crash the car it is Adam killing off the memories , squashing the spider per Se , this is supported when it zooms in on the crashed cars window the crack looks like a spiderweb, when Adam wakes up the radio speaks of a car crash but it says no details so It is likely that was just chance. also Helen reminds him about his mother calling him, which you should remember from the beginning of the movie. when Adam opens the letter and gets the key, it is a key to the strip club place, and he says to Helen he might have to go somewhere tonight. he goes to look at her and she is a huge spider, this represents how him going to the strip club would bring back the spider. during the movie the spider gets gradually bigger until it is huge and he cannot get it off his mind. so I hope I've explained well enough to help you understand what an amazing movie this truly is.