The true-life story of Megan Leavey, a young Marine corporal whose unique discipline and bond with a military combat dog saved many lives during her deployment in Iraq. Assigned to clean up the K-9 unit after a disciplinary hearing, Leavey starts to identify with Rex, a particularly aggressive dog that she trains. Over the course of their service, Megan and Rex complete more than 100 missions until an improvised explosive device injures both, putting their fates in jeopardy.