USMC officer Sam Cahill is about to be sent to war in Afghanistan in 2007. He is married to his high school sweetheart Grace Cahill, Connie Nielsen, with whom he has two young daughters, Isabelle and Maggie. Sam's younger brother, Tommy, is released from prison on parole after being arrested for armed robbery, just a few days before Sam's departure. At a family dinner with Hank and Elsie, the Cahills' parents, Hank insults Tommy for not understanding his duty and for his criminal past.
During Sam's tour, his helicopter is shot down, and he and Private Joe Willis are the sole survivors. They are taken prisoner by the Taliban, but are declared killed in action by the US government. Upon hearing the news of his brother's presumed death, Tommy berates Grace for letting Sam go to Afghanistan. At Sam's memorial service, Hank attempts to drive Elsie and the girls home while drunk, but Tommy intercepts him. Hank once again berates Tommy, and Tommy accuses Hank of influencing Sam to join the Marines because of his own Vietnam War service.
Tommy helps in completing a kitchen remodel for Grace. He attempts to mend his relationship with Hank, and he bonds with Grace, aided by his growing paternal connection with Isabelle and Maggie. Tommy also apologizes to the bank teller who he held at gunpoint during the robbery. Grace and Tommy unwittingly kiss, but do not take their attraction any further. However, Tommy continues to remain close with the family and his nieces grow attached to him. Meanwhile, Sam and Joe are tortured by their captors and Sam is eventually forced to brutally beat Joe to death with a metal pipe.
Several months later, Sam is rescued and returns home, where he struggles to readjust, showing signs of severe post-traumatic stress disorder; his daughters grow fearful and resentful toward him. Sam also lies to Joe's widow about her husband's death. His paranoia also causes him to believe Grace and Tommy fell in love while he went away. When Grace questions Sam about what happened in Afghanistan, Sam does not answer and demands to know what happened between her and Tommy. Grace tells Sam that she and Tommy shared only one kiss while he was gone, but Sam has trouble believing that is all that happened.
During Maggie's birthday party, Tommy shows up with a woman, Tina. While Isabelle plays with a few of Maggie's new toys, Isabelle complains that she never got what she wanted for her birthday, and that Sam was also absent on her birthday. She is also irritated by the reprimand of her parents and the attention shown to Tina, then begins to play with a balloon loudly, causing Sam to become irritated and annoyed and pop it out of rage in front of the family. A hurt Isabelle falsely claims that Tommy and Grace are having an affair and angrily tells Sam that she wishes he had actually died.
Returning home, Grace puts the daughters to bed. Isabelle apologizes to Grace, saying that she prefers having her Uncle Tommy around over their own father. Grace assures both of the girls that Sam will get better soon. Grace then calls Elsie, concerned for their safety. Tommy prepares to go over to their house as Elsie urges Hank to call the police. Sam, believing his daughter's story and angry about killing Joe in order to get back home, loses his temper and angrily wrecks the kitchen with a crowbar. He briefly comforts when Tommy shows up and proceeds to embrace him, in order to calm his brother's furious breakdown. However, Sam becomes defensive and holds Tommy at gunpoint. DOD Police arrive and confront Sam, leading into a standoff. He fires a pistol into the air and demands that the policemen kill him. Tommy quiets the officers and attempts to talk to Sam, but Sam puts the gun to his own head, before tearfully saying to Tommy that he's "drowning" and he surrenders.
Sam is arrested and admitted to a Veterans' Affairs hospital. After his arrest, Grace reads the letter Sam wrote her, to be delivered in the event of his passing. Eventually, Grace visits him, giving him an ultimatum that if he does not tell her the truth, he will lose both her and his family forever. Sam finally confesses that he killed Joe and they embrace. Devastated and traumatized, he wonders if he will ever be able to live a normal life again.
