In the 1960s, 14-year old Walter Coleman (Haley Joel Osment) goes to live with his bachelor uncles, who are rumored to have a secret fortune. When they first arrive, Hub and Garth McCann (Robert Duvall, Michael Caine) are knee-deep in their pond, shooting at bass with shotguns. His cousins, also strong believers in the fortune, arrive a day after his mother leaves to ingratiate themselves into the good graces of Hub and Garth. They scare Walter away, and he runs to telephone the court reporting school where his mother had claimed to have gone. He discovers that she is not actually enrolled at the school, but has lied — again. With no recourse but to return to his uncles' farm, Walter eventually becomes used to their quirky, socially atypical mannerisms and they become used to him and his shyness.
While encouraging them to eat vegetables and stop shooting at visiting salesmen, he becomes aware of his uncles' mysterious past. Garth reveals to him that he and Hub were adventurers beginning in World War I. Hub and Garth arrived in France the day that Germany invaded the country. Garth believed they should return home, but Hub proposed that they tour Europe just ahead of the invading armies. On their last night in France, after enjoying the local nightlife in Marseilles with two French soldiers, they find themselves shanghaied on a ship, conscripted into the French Foreign Legion. The two brothers fought in many battles, and Hub saved Garth's life several times, in many different battles. Following their army service, Garth became a guide in Africa, while Hub helped anybody who deserved it. Hub meets and falls in love with Jasmine (Emmanuelle Vaugier), a beautiful princess. She, however, was promised to be a powerful sheik's wife. Hub rescues Jasmine from the sheik, who then puts a bounty of ten thousand gold pieces on Hub's head. Garth, disguised as a bounty hunter, turns Hub in for the reward, whereupon Hub duels the sheik and wins. Having earned the sheik's respect, the assassination attempts stop. Later, the sheik makes his fortune in trade of petroleum.
Meanwhile, Hub, Garth and Walter have adventures of their own. They order an aging lioness from a circus animal dealer, whom Walter adopts and names Jasmine, thereby keeping Hub from killing her. Hub and Garth frighten off Walter's cousins and purchase a clay pigeon thrower, a succession of ever-larger boats, and a biplane. Hub gets into a fight with several greasers within an hour after having a heart attack, wins the fight, drives them back to the farm, then feeds them (after hunting for the lioness, which escaped), and gives them a lecture on what it means to be a real man. Walter subsequently finds that his uncles have been hiding their money in a room beneath their barn.
Fresh from Las Vegas (where she had been instead of at the school), Walter's mother and her current suitor named Stan (Nicky Katt), arrive at the farm and demand to know where the money is. Stan tells Walter that Hub and Garth actually were bank robbers, that Jasmine was their accomplice, and the fortune is for the taking. Walter decides he will believe in the tales of his uncles, and Stan begins to beat Walter. Jasmine (the lioness) attacks Stan and seriously wounds him, but the lioness dies of a heart attack in the process. The next day, Walter is taken away by his mother, but after she informs him that she has no intention of leaving Stan, he jumps out of the car. In an emotional argument, Walter asks her to "do something for me for once" and she lets him return to his uncles' house. He gives them an ultimatum that they need to be around through his high-school years (or better yet college) and be there to give him that 'how to be a man' speech.In the film's epilogue which takes place several years later, Walter (Josh Lucas), now a successful cartoonist whose comic strip ("Walt and Jasmine") is based on his experiences with his uncles, returns back to the farm after learning that his uncles have flown their biplane into their barn and died. The sheriff hands their will to Walter and as he reads it ("The kid gets it all. Just plant us in the damn garden with the stupid lion.") and laughs to himself about his uncles' unique approach to life, a helicopter touches down and a Middle Eastern man steps out. He explains that, while in Texas tending to his oil interests, he heard of the men's deaths on the news and thought they might be the two courageous Americans that his grandfather (the sheik) had told him about long ago. Walter says that, yes, they were, and the two are happy to realize that the stories which had inspired them for so long were true—that the two men "really lived".