Lincoln Hawk, Sylvester Stallone, is a truck driver who also arm wrestles for extra cash. His estranged wife Christina, Susan Blakely, who is suffering from heart disease, asks that Hawk pick up their young son Michael, David Mendenhall, from military school and develop a relationship with him (Hawk had left them ten years earlier). Michael's wealthy grandfather — Christina's father, Jason Cutler, Robert Loggia, believes that Hawk has no right to be in his grandson's life. Michael distrusts Hawk initially and treats him with contempt.
Over the course of a trip from Colorado to California, the two bond. When they arrive at the hospital, Christina dies from complications during surgery. Michael blames Hawk for delaying his arrival and leaves for his grandfather's estate. An attempt to retrieve Michael ends with Hawk being arrested when he resorts to ramraiding the gated mansion. Michael visits Hawk in jail and forgives him but says he feels safer living with his grandfather. As a condition of charges being dropped, Hawk is obliged to sign over custody of Michael to Cutler.
Hawk leaves to compete in the World Armwrestling Championship in Las Vegas, hoping to start his own trucking company with the prize money. Most other participants are much larger, including Bull Hurley, Terry Funk, the undefeated world champion for the past five years. When Hawk arrives, he sells his truck and uses the proceeds to place a bet on himself (as a 20–1 long shot) to win. Meanwhile, Michael learns that his grandfather had driven his parents apart and had been hiding letters Hawk had regularly written to him. Stunned by his grandfather's deceptions, Michael steals a pickup truck and drives to Las Vegas to find Hawk.
Hawk advances to the final eight competitors in the double-elimination tournament before suffering his first loss, injuring his arm in the process. Cutler, who is also in Las Vegas, summons Hawk to his hotel suite and offers him a chance for a fresh start: $500,000 and a top-of-the-line semi on the condition that he stay out of their lives for good, but Hawk refuses and vows to retrieve Michael after the tournament. He returns to the tournament with improved focus and advances to the final match against Hurley. Michael finds Hawk and apologizes for misjudging him, which gives Hawk the emotional support to compete. After a long match, Hawk beats Hurley and wins the tournament. A triumphant Hawk and Michael take their winnings and drive off to start a new life together.
