The film is based on the life story of a young Marine Corporal, Megan Leavey (Kate Mara), whose unique discipline and connection to her military fighting dog has saved many lives during their military troops in Iraq. Megan is a young woman who enrolled in the army to run away from her own life and to hide from her unhappiness. When she is assigned to order in the K9 unit after the disciplinary hearing, Leavey identifies a particularly aggressive dog and gets the chance to train him. Megan forms a strong and special relationship with Rex, an aggressive fighting dog, which he trains and disciplines. During their military service in Iraq, Megan and Rex have completed more than 100 missions and saved countless lives, but an IED explosion hurts them, putting their fate in jeopardy and fidelity to the test. When the dog ends up on a list to be euthanized, Megan prepares for the fight of her life to bring her 11-year-old German shepherd home safely. Her most important mission is to bring home the hero dog that saved her life and taught her what love means.