Dr. Claire Summers (Anne Hathaway) is a therapist who is assigned by her mentor to council the five survivors of a plane crash.  Each of the survivors have differing views as to what happened, but most of them believe that there was an explosion on the plane.  The airline claimed that the cause of the crash was pilot error, not an explosion.  One of the survivors, Eric (Patrick Wilson), beings showing symptoms that make Claire believe he is not telling the truth concerning his true feelings.  As she delves deeper into Eric's feelings, they eventually begin a romantic relationship, even though she knows that's against the rules of her profession.  At this time, some of the survivors begin disappearing, while the others and Claire begin to notice that they are being followed.  Claire begins to suspect that the airline is responsible for the disappearance of the survivors.  The airline has a history of mechanical problems, and another incident would be the end of the company.  Claire then begins her own investigation of the crash.

One night, Eric suggests to Claire that she make peace with her estranged sister Emma.  Reluctantly, she agrees, and goes to her sister's house.  She is not there, but the man who has been following Claire and the other survivors is there.  He tells her to stop investigating the crash because it really was pilot error and there were no survivors.  He leaves her his briefcase, which includes a passenger manifest.  Claire then discovers her name on the manifest, and suddenly remembers everything.  She was a passenger on the doomed flight, and she was killed in the crash.  Everyone who she has come into contact with since the crash, including Eric, were dead.  They were friends and family from her life (and were not recognized by Claire until she remembered everything) who have passed on and are trying to help her come to grips with the fact she is dead and to make a transition to the other side.  All of the survivors who had disappeared had come to grips with their situation and went to the other side, and the man who was following them was the pilot of the plane.

The final scene of the movie shows Emma and her husband at Claire's apartment.  There, she sees the letter that Claire had meant to give to Emma once they met.  Belatedly, Claire and Emma had made their peace.

Thanks Steve!