Captain Sam Cahill (Tobey Maguire) is a hard-working Marine who has just received yet another order to ship out to Afghanistan. Before he leaves, he picks up his brother Tommy (Jake Gyllenhaal) after he just finished serving a term in prison for a bank robbery. He hopes that Tommy will be able to grow up and be there for Sam's wife Grace (Natalie Portman) and their two children. When Sam's plane is gunned down and he is feared dead, Tommy steps up and becomes a rock for Grace and the children, and Grace and Tommy find themselves romantically involved. When Sam is rescued and brought home, his adjustment back to civilian life is proving to be challenging, and is made even more difficult when he learns of Tommy's relationship with Grace. Will Sam's extreme jealousy tear the family apart?