Three years after being left for dead behind enemy lines in Ethiopia where his spotter partner was killed, military sniper Bob Lee Swagger (MARK WAHLBERG) leads a quiet life in the woods with his faithful dog. Then, out of the blue, Col. Isaac Johnson (DANNY GLOVER) and his small entourage, including Jack Payne (ELIAS KOTEAS), show up with a request. They claim they've discovered an inside plot to kill the President of the U.S. and thus need Swagger's expertise in the field to catch the shooter before he commits the act.
The former gunnery sergeant is reluctant, but the call to serve his country is too strong. Accordingly, he ends up in Philadelphia where the President is scheduled to make a public appearance with the Ethiopian Archbishop. Before he knows it, the latter is shot dead, and a local cop then shoots and wounds Swagger who nevertheless manages to escape, but not before running into rookie FBI agent Nick Memphis (MICHAEL PENA).
Realizing he's been set up as the assassin, Swagger tends to his wounds and, with nowhere else to turn, finds his former partner's widow, Sarah Fenn (KATE MARA), for help. As Nick discovers inconsistencies regarding the belief that Swagger is the perp, the sniper sets out both to clear his name and to get revenge on those who've wronged him, including Johnson, former sniper turned operative Michael Sandor (RADE SERBEDZIJA), and corrupt U.S. Senator Charles F. Meachum (NED BEATTY).