Michael Sullivan (TOM HANKS) is a Depression era hitman who works for mob boss John Rooney (PAUL NEWMAN) who took him in years ago and raised him just like his own son, Connor (DANIEL CRAIG), now the number two man in the organization. Michael is a loyal and effective worker who's now a family man himself, raising sons Michael Jr. (TYLER HOECHLIN) and Peter (LIAM AIKEN) with his wife Annie (JENNIFER JASON LEIGH).
The two are careful not to allow the boys to learn what their father does for a living, but one night Michael Jr. sneaks out to watch his father and ends up seeing him mow down some people after Connor abruptly executes an associate. Everyone in the organization is understandably concerned about the 12-year-old being able to keep quiet about what he saw, but Connor decides to take matters into his own hands.
Ordering a hit on Michael, the lose cannon sets out to kill Michael Jr. as well, but instead ends up murdering Annie and Peter. When Michael Sr. discovers this, he realizes the world he once knew no longer exists and thus heads to Chicago to meet Frank Nitti (STANLEY TUCCI), Al Capone's right-hand man. Michael proposes that he join Nitti's organization in exchange for them turning a blind eye to his pending revenge, but Nitti says he can't help.
Instead, and with the approval of John who has to make the toughest decision of his life in favoring his biological son over his adoptive one, Nitti hires Maguire (JUDE LAW), a press photographer and hitman, to find and kill Michael before things get out of hand.
Michael has other plans, however, and thus sets out to get his revenge, any way he can, on those who wronged him, all while protecting and ironically finally getting to know his son.