The opening twenty minutes try to establish the character of Sam. We understand he has blackouts, he is sleeping with a girl and he is a bit awkward, but he somehow remains a blank slate. He's so bland you wonder why the girl was interested in him in the first place.
We find out that the girl he was sleeping with is the sister of his work partner's wife and she was already married to a guy coming home from war. The husband, Tyler, is understandably pissed off when he finds out, though he manages to figure out by seeing them kiss for a split second. The wife, Mia, is giving our main lead Sam the cold shoulder and this is too much for our wooden protagonist, so he leaves the next morning after creeping the pair in the bedroom while they sleep.
Tyler follows him and beats him senseless, eventually killing him in what could be an accident, we never learn that. One thing that this movie does do well is the shots of the soul leaving the body, where it seems to slip back from the eyes and flutter over the scene. But it's all downhill from here.
Sam wakes up in the body of Jay, how work partner, and after freaking out his wife, Poppy, as she tries to give him a birthday blowjob, he decides his best plan is to confront Tyler about killing him. Tyler, for some reason, actually believes Sam is inside Jay and confronts him. Sam continues to poke and prod at Tyler until he kills Sam...or Jay, which leads to the question: where did Jay go? Did he just blink out of existence?
Sam then ends up in Poppy and even after it going so horribly the last time, he goes right back to antagonizing Tyler. And if Jay is now a disembodied spirit and knows what happened to his body and Sam, wouldn't he fight to come back to protect his wife? Somehow Sam convinces Mia that he is now in the body of her sister and they bolt, but then take a detour to where Sam was killed and sit around and talk, allowing Tyler to catch up. They escape him once more, ending up with Sam in the back of Tyler's truck, where he puts an ax through the back window, which causes Tyler to crash into a lake and the truck sinks. Poppy's body becomes trapped and she apparently dies.
Sam then wakes up in Tyler, who has somehow survived an ax to the back and a car crash after going airborne into the water, but convinced his spirit to leave his body. Tyler's body is also still able to get out of the truck, free Poppy's body, carry her to the surface and revive her with CPR. He is then allowed to finally die because he saved someone, after not being able to save his sister from falling off a cliff and drowning as a child.
But then, in the final flashback of the movie (they are peppered through the movie), it shows his sister passing away and her spirit hovering around him, inferring that she was inside him the entire time, which ties back into his weirdness in the first twenty minutes of the movie.
We understand that Sam feels horrible that he couldn't protect his sister, but he is a paramedic. He has undoubtedly saved some people's lives in his career. He tells Mia that she was his first sexual encounter, which only adds to his weirdness, given he is mid to late twenties. But he is overly infatuated with Mia and yes Tyler is an absolute basket case, but the movie stretches his motivations to continue coming back three times to protect her.
If Sam and Tyler took two seconds to think, they'd realize that neither of them have to follow the course they go on. Sam could "hide out" in Jay and then send police to find his body and implicate Tyler, but he continues to antagonize him. Tyler should realize that no one is going to believe (at least at first) that Sam is inside Jay and take the time to get rid of Sam's body.
Jay and Poppy literally get screwed over because the silly love triangle goes crazy. Does Poppy remember being possessed by Sam? What kind of story do the sisters come up with to explain Jay's dead body, thinking that they explain Sam and Tyler killed each other?
See what I mean about too many questions.