Arriving unannounced on the doorstep of the Peterson family, recently discharged soldier David (Dan Stevens) brings a final message of love from their recently deceased son, Caleb, with whom he served. Still grieving over Caleb's tragic death in combat, the Petersons welcome David into their home and their lives. Humble, helpful and unfailingly polite, David turns out to be the perfect guest, filling the void left by Caleb. He consoles Caleb's mom (Sheila Kelley), helps dad Spencer (Leland Orser) get ahead at work, teaches teenager Luke (Brendan Meyer) how to defend himself against bullies and rids rebellious daughter Anna (Maika Monroe) of her loser boyfriend. But David's unorthodox problem-solving methods begin to raise Anna's suspicions as a wave of shocking and unexplained violence spreads through their small town. When her inquiries into his past raise a red flag with a super-secret private military organization, it sets the stage for a deadly final conflict.