Nat really does have a brain tumor and is able to patch up his relationship with his son Ritchie before he dies. The film ends with Ritchie and Maggie spreading Nat's ashes into the lake outside the lake home he left to them, though the small boat he also left them sinks as they pour the ashes into the lake. The two laugh and swim to shore.
Flynn (Alessandro Nivola) works as a health inspector in California. One day, his boss fires him after receiving an anonymous letter notifying them that Flynn has a criminal record. When he goes home, he finds that his girlfriend Maggie (Amanda Peet) is leaving him. Maggie got a call from Flynn's dad (who Flynn had claimed was dead) asking for "Ritchie." Flynn admits his name is "Richie" and that "Flynn" is his middle name. He also states that his dad is a lifelong con-artist, that they used to pull con jobs together, and that Richie had been arrested taking the fall for one of father's crimes. Maggie is disgusted at Richie's duplicity and leaves, but not before telling him that his dad is dying and wants to see him.
Having nothing left, Richie flies out to New Jersey where his dad is living rent-free at a carnival. His father, Nat (Christopher Walken), tells Richie he is dying of a brain tumor and needs Richie to drive him to New Mexico for alternative treatment (he can't fly due to the tumor). It turns out that Nat is still pulls small con jobs, making a living while spending only $5.00 a day. Nat has the journey set out, whereby he and Richie would swing by various cities that have sentimental meaning to the duo. Though Richie is reluctant, he agrees, and they set off in a car covered in "SweetnLow" advertisements (accordingly, the car and gas are free, courtesy of the sponsor).
Though Richie is still unhappy with his dad for letting him go to jail and chasing away their mom, the two begin to bond over their shoestring living. They stop at one place to visit Richie's old babysitter Dolores (Sharon Stone) who Richie had a crush on. Though Dolores initially flirts with Richie, he realizes that she is actually in love with (and had learned how to con people from) Nat. Dolores reveals that in actuality, Richie's mom had an affair with Nat's business partner Burt (Peter Coyote) and that is the real reason the family fell apart. This is doubly shocking since Nat had loaned Burt the money to open his first auto dealership, and now Burt owns a fleet of dealerships and is incredibly wealthy.
Richie deduces that they are stopping at Burt's hometown for revenge. Nat confirms Richie's suspicions, but says he wants to see Burt alone. When the two men meet, Nat tells Burt he is dying and that Richie is actually Burt's son stemming from his affair. Nat knows that Burt is announcing his candidacy for mayor that afternoon and will keep the secret quiet as long as Burt pays him $150,000 so that Richie can have a "nest egg." Nat springs for a nice hotel room and tells Richie about his scheme, saying he actually has no idea who Richie's real dad is. Nat also confesses that he falsified the tumor. Richie storms out to return the money and confront Burt. When they meet. Burt says he is happy to explore being Richie's dad after the election. Realizing that regardless of biology, Nat is a true father, Richie return most of the bribe money and publicly exposes Burt's affair to the press. When Richie returns to the room, however, he sees his dad has passed out. Nat is hospitalized and we learn that Nat really does have a tumor and only told Richie what he wanted to hear.
Throughout the journey Richie has been leaving confessions about his life on Maggie's voicemail. Upon hearing Nat is really near the end, she forgives Richie and flies out to help he and Nat finish their journey. When they reach New Mexico, they find a small lake house and old boat that Nat had won in a poker game and wants to leave to Richie. Knowing there is no cure or treatment for Nat, the three enjoy their days at the house until Nat passes away. The film ends with Maggie and Richie pouring Nat's ashes over the lake, but then the old boat begins to sink. The two laughingly swim to shore.