After attending Grandparents' Day at school and embarrassing Bart in front of his class, Grampa learns that a member of his old army unit has died. Now, either he or Mr. Burns, who also served in the unit, will inherit a fortune from the past--depending on which one survives the other. After the funeral, Burns plans to have Grampa killed.

Following three attempts on his life, Grampa seeks refuge at the Simpsons' home. He tells Bart the story of his old platoon, the Flying Hellfish: during the war, that members of the platoon made a pact to determine which one of them would eventually own the priceless art that the unit stole from the Nazis. Each one received a key to a special safe. As each member died, his key was added to the safe's giant lock. The last surviving member would own the treasure. Bart refuses to believe Grampa until Burns tries to steal Grampa's key by forcing his way into the house on a mechanical platform. Bart manages to get both the keys in his possession.

Together, Bart and Grampa set out to capture the booty at the bottom of Lake Springfield. However, as soon as they recover the case of masterpieces, Burns steals it. Grampa and Bart give chase, and a fight ensues between Grampa and Burns, which is broken up by representatives of the State Department, who have arrived to return the paintings to their proper heir. Although Grampa has lost his chance of inheriting a fortune, Bart is proud of him.