Determined to build a life for herself after her divorce, single mother Caroline Whitney agrees to go on vacation with her boyfriend, Jean-Pierre, a French banker. A week before the new school year, she sends her twelve and seven-year-old sons, Josh and Sam, to stay with their father, Thom’s, new family in Florida. Josh stubbornly complains about the arrangement and makes fun of his quiet and antisocial younger brother for having to repeat the second grade, deepening the rift between them. Once the boys arrive in Florida, Thom Whitney criticizes Josh for not being athletic like his two stepsons, who tease Josh for being sensitive. That night, Josh finds a Time magazine article about “child warriors” in Burma, and devises a story to explain Sam’s strange behavior. Insisting “Sam” stands for “Strategically Altered Mutant,” he explains that the boy was genetically modified and enlisted by their parents to serve as a soldier for an imaginary war in Africa. Later, Thom announces that their mother has agreed to move to Europe with Jean-Pierre for a year, leaving the boys to live with their father. Distraught, the boys return to California to gather their belongings. On the airplane, Josh informs Sam he will be ready to report for military service once they find a beautiful woman known as the “Liberty Maid,” who recruits child warriors and takes them to Canada for training. When a storm forces their plane to land in Dallas, Texas, the boys spend the night in a hotel. Josh attempts to sneak out, but gets caught in the middle of a high school reunion party gathered in the lobby and pretends to be the illegitimate son of an absent attendee named Michelle Geary. His story attracts the attention of Michelle’s intoxicated former lover, Derek Baxter, who suspects he may be the boys’ father and offers to take them to dinner. Upon realizing that Josh is lying, he becomes violent, and Josh knocks him unconscious. Assuming Derek is dead, they steal his rental car and drive until they must stop to rest. Although Josh admits he lied about Sam’s special abilities, the younger boy refuses to believe him and insists they go to Canada anyway. In the morning, Josh awakens to find Sam behind the wheel, driving top-speed through an orchard. Josh convinces him to switch places, and they continue toward Colorado. The next day, Sam convinces Josh to pick up a hitchhiker because she matches Josh’s description of the “Liberty Maid.” The woman agrees to drive and plays along with Sam’s outlandish story. At a gas station, Josh uses a payphone to call his mother, but Jean-Pierre answers and refers to Caroline as his “wife,” indicating that they have recently married. Furious, Josh uses stolen copies of Jean-Pierre’s debit cards to withdraw money from an automated teller machine and buys them all dinner. Grateful, the “Liberty Maid” reveals that her real name is Alison, and she is saddened to learn that Sam has deluded himself about being a child assassin in order to justify all the negative things in his life. Near Salt Lake City, Utah, a passing police officer becomes suspicious and orders the boys to accompany him to the station so he can call their parents. Sam flees, crawling under a passing freight train and running back toward the car. Josh and Alison catch up to him, and they drive to the next town. While stopped at a Laundromat, Alison pretends to receive an important telephone call from the U.S. Department of Defense, informing her that the war in Africa has ended. Sam suspects she is lying to get rid of him and steals the car. Alison asks Josh to accompany her to Seattle, Washington, but he decides to stay and search for his brother. Once Alison leaves, he finds the car crashed on the side of the road, but Sam is not inside. Unable to start the car, Josh boards a bus toward Canada. On the highway, the bus passes a semi-trailer truck, and Josh notices Sam seated on top of the cab. At the border crossing, Josh retrieves his brother and they sneak under the fence into Canada. While eating in a diner, Sam apologizes for causing his brother so much trouble. In the morning, Josh suggests Sam return to Florida and takes him to a tanning salon, where he pays the owner to pretend the ultraviolet lights emit radiation that will “reverse” Sam’s genetic modifications. While Sam lies in the tanning bed, Josh uses the owner’s typewriter to falsify a letter stating that Sam’s parents have just died in a propane gas explosion. At the airport, Josh presents the letter to the gate agents and secures Sam a free ticket on the next flight. Although the boys have bonded over the course of their journey, Josh insists he cannot bear to live with his father, and must continue on his own. They share a solemn goodbye, and Sam boards his flight. While wandering the streets alone, Josh calls Derek Baxter and discovers he is still alive. Reunited with his father in Florida, Sam asks Thom why he does not love Josh. Thom says he regrets being unable to give his sons a happy family life and thought that forcing Josh to be “tough” would help him overcome his hardships. Josh eventually returns to Florida, and Sam confronts him for lying about his special abilities. Josh apologizes, and they hug.