Bart is mortified when Grampa speaks before his class as part of Grandparent’s Day. Grampa tells students that he invented “the terlet.” Later, Grampa receives word that the seventh member of his old army unit has died. At the funeral, he and Mr. Burns argue over which of them will outlive the other—and thereby inherit a fortune in rare art. When Burns returns to his limo, he tells Smithers he wants Grampa killed.

True to his word, Burns hires the world’s most devious assassin to eliminate his rival. But try as he might, the killer’s schemes to kill Grampa fail. Certain that someone is out to get him, Grampa moves into the Simpson home. Bart, however, is certain his grandfather’s story about a murder plot is simply another wild tale. To prove his story, Grampa tells Bart about his adventures during World War II when he commanded the Flying Hellfish, the toughest battalion in the army. One of the privates in his unit was none other than Montgomery Burns. During a battle, the Hellfish captured a German castle filled with priceless rare art. Burns convinced his fellow soldiers to enter a contract whereby the artwork was hidden by a third party—and the last surviving participant would become the sole possessor. Bart dismisses the tale as fiction—until Burns bursts into the house in search of Grampa’s treasure key. Bart gets the upper hand, however, and steals Mr. Burns’ key.

Bart and Grampa decide to locate the treasure before Burns beats them to it. They realize the artwork is buried beneath Lake Springfield. Bart dons scuba gear and raises a crate containing the sunken treasure. But Mr. Burns, gun in hand, confronts his rivals. He locks Bart in the crate and kicks it overboard. Grampa rescues his grandson, then catches up to Burns and overpowers him. But before anyone can make off with the treasure, State Department officials appear and claim the missing art must be returned to the descendant of its rightful owner.