Jerry Ferro is turning 40 years old and leading a mediocre life. He works part-time in construction as a carpenter and part-time as a boxing instructor at a fitness center near Los Angeles. He drives a beat-up pickup truck and loses both the construction job and the woman in his life.
An amateur Golden Gloves boxer long ago, he is asked one day if he would be willing to spar with an up-and-coming fighter named Malice Blake. He takes a beating, but surprisingly lands a one-punch knockout. Due to his strong left hand, Jerry's boxing nickname is “The Hammer”, a running joke that suggests the name fits his carpentry more than his punching power.
He is noticed by Eddie Bell, a coach who will train boxers trying to qualify for the next Olympic Games. Bell invites him to come to the Olympic tryouts. Jerry is clearly too old and out-of-shape, so he is not interested. But his friend and co-worker Oswaldo Sanchez gives him encouragement, as does Lindsay Pratt, an attractive lawyer who works out at the gym and has caught Jerry's eye.
Jerry comes to the tryouts. He is mocked by Bell's top light-heavyweight contender, Robert Brown, which motivates Jerry to get into shape. He goes into serious training, loses weight and needles Brown, telling him there is > "a new sheriff" in town.
What Jerry does not know is that Coach Bell privately sees Jerry as no more than a sparring partner and motivator for Brown, who is a legitimate Olympic candidate but who has a weakness against hard-hitting left-handers like Jerry. By falsely boosting Jerry's hopes, Bell can avoid paying him the usual $100 per day for a hired sparring partner.
Jerry drives team members to the first stage of the Olympic qualifying eliminations in Phoenix, where because of his age and lowly status he is immediately paired against the No. 1 contender. Bell has gotten all he wanted out of Jerry and is so apathetic about the outcome that he deserts Jerry's corner in the middle of the bout. Motivated by the memory that he has never finished anything he ever started, Jerry scores a shocking knockout.
He and Brown both advance to the next round of the competition, to be held in Pasadena. When Brown learns that Bell has lied to Jerry about his prospects, Brown begins to sympathize with Jerry and to bond with him as unlikely friends. Unfortunately, they will need to fight each other soon, with only one able to qualify for the Olympics.
On top of that, Lindsay has a job offer that would require her to move out of state. Jerry knows he has little future in boxing, no matter what happens next, so he needs to get his priorities straight as they head toward a showdown in the ring.
In the final round of the competition, Jerry and Brown go up against each other. Lindsay decides to stay in California. At the fight, right as Jerry is about to win and KO Brown, Jerry instead hugs him and lets Brown win. The film ends with all three at a party at their house and Jerry becoming Brown's trainer.
