The relationship between Grampa and Bart deteriorates after Grampa's senility and abrasiveness embarrass Bart during Grandparents Day at Springfield Elementary School, where Grampa claims to have invented the toilet, to have turned cats and dogs against one another, and that Kaiser Wilhelm II stole the word "twenty".
Back at the retirement home, Grampa receives word that Asa Phelps, one of the men who served under his command in the Army during World War II, has died. Their infantry squad – known as the "Flying Hellfish" (despite not being an aviation unit) – also included Montgomery Burns, Chief Wiggum's father Iggy, Seymour Skinner's father Sheldon, Griff McDonald, Milton "Oxford" or "Ox" Newman-Haas, Etch Westgrin and Barney Gumble's father Arnie. (It is also mentioned that their field commander was a Flanders, though as his father is portrayed as beatnik in other episodes, this is perhaps Ned's grandfather or uncle.) Grampa and Mr. Burns are now the only two surviving members of their squad. Burns wants what he describes as the "Hellfish Bonanza" for himself; not wanting to wait for Grampa's natural death, he hires Fernando Vidal, "the world's most devious assassin," to kill him. Vidal tries several tricks (including disguising himself as Homer, with Burns as Marge and Smithers as Bart), and as a last resort storms Grampa's retirement home with a machine gun. Grampa evades the initial volleys by sheer luck and escapes when the attending nurse produces a shotgun and routs the surprised assassin. Fearing that Burns will not give up, Grampa takes shelter with the Simpsons. He moves into Bart's room, putting further strain on their relationship, and reveals to Bart his reason Burns wants him dead.
In a flashback, it is revealed that during the final days of World War II, the Flying Hellfish discovered several priceless paintings in a German castle. To avoid being caught for theft, the soldiers set up a tontine and locked the paintings in a strongbox to be hidden away; the one who outlived all the others would then inherit the collection. Each man was given a key, all of which would eventually be needed to trigger a mechanism that would reveal where the paintings were hidden. Ox is the first to die, from a hernia he got from carrying the box out of the castle. The majority of the other soldiers died in an unspecified accident at a Veterans Day parade in 1974, with Asa being the last to die other than Burns and Grampa.
After Grampa finishes his story, Bart dismisses it as fiction but is proven wrong when Burns suddenly breaks in to his room and tries to take Grampa's key by force. However, Bart manages to steal both keys back and the two Simpsons go after the paintings. They hurry to the local cemetery, where a monument to the Hellfish is located. After activating the locator mechanism built into the monument, they discover that the paintings have been hidden at the bottom of a lake, so they steal a motorboat from Ned Flanders and head out onto the water. Bart brings up the strongbox, but just as he and Grampa open it, Burns shows up to take the paintings at gunpoint. When Bart calls him a coward and an embarrassment to the Hellfish, Burns kicks him into the empty strongbox, which topples back into the lake. Grampa dives in to save Bart, and the two chase Burns back to shore; Grampa then overpowers Burns, gives him a dishonorable discharge for trying to kill his commanding officer and his grandson, and expels him from the tontine, much to Burns' distress.
Before Grampa and Bart can go home with the paintings, representatives of the U.S. Department of State arrive on the scene. The government has been trying for 50 years to track down the paintings. Rather than arresting Grampa and Bart for the theft, the representatives instead turn over the paintings to a Euro-trash heir of one of the original owners in order to avoid an international incident, and Bart and Grampa are left empty-handed as the heir leaves for a Kraftwerk concert in Stuttgart. Grampa tells Bart that he did all of this to show that he used to be something besides a pathetic old man, and Bart says that he never thought of Grampa that way. The two hug, their relationship back on good terms.