The relationship between Grampa and Bart deteriorates after Grampa's senility and abrasiveness embarrasses Bart during Grandparents Day at the elementary school. At around the same time, one of Grampa's fellow-veterans of the Second World War dies, leaving Mr. Burns and Grampa as the only other living members of Abe's war squad, the Flying Hellfish. In the final days of the war, the unit had removed Germans from an "abandoned" castle when Burns discovered several paintings. The group had agreed on a tontine, placing the paintings in a crate, and each member received a key. When all but one of the members died, the final and surviving member would inherit the paintings. As Mr. Burns wants the paintings as soon as possible, he orders Grampa's assassination and hires an assassin to do the job. Fortunately, Grampa escapes from the assassin.
To escape death, Grampa moves into the Simpsons house, where the family lets him live in Bart's room, putting a further strain on the relationship until Mr. Burns bursts in to take the last key to the paintings' safe. Bart manages to not only keep Grampa's key but also to steal Mr. Burns's key as well, and the two Simpsons go in search of the paintings. As it turns out, the safe is buried in the lake, so they steal Ned Flanders' boat (Which is ironic because a relative of Flanders was their commander although it is never mentioned how they are a related and is never shown) and use it to head out into the lake to obtain the valuable works of art. Bart manages to bring it out of the lake and Grampa manages to open it up to get the paintings, but Mr. Burns again shows up to retrieve the paintings. Bart angrily calls Burns a coward and an embarrassment to the name Hellfish, and in response, Burns kicks Bart into the lake and escapes. Grampa rescues Bart and gives chase to Burns, and after the boat hits land, Grampa, who had been Mr. Burns's superior in the war, gives his rival a dishonorable discharge for trying to kill his commanding officer, much to Burns's dismay. Grampa then announces the paintings are his by default until the US State Department shows up. Rather than arresting either Grampa or Burns, the State Department instead takes the paintings, having been trying to find them for 50 years to return them to their rightful owner in order to prevent an international incident with the German government. The "rightful owner" is a rude young German, Baron Herzenberger, who is meant to be a descendant of one of the German politicians during World War II. The State Department then get the paintings carefully dumped into his car's trunk and then he quickly leaves for a Kraftwerk concert in Stuttgart. Grampa then tells Bart that he did all of this to prove that he wasn't always a pathetic old kook. Bart says that he never was, pushing their relationship back on good status, and they end up hugging each other.
Burns was always the unit's troublemaker. The Hellfish got stuck with Burns because he obstructed a probe from J. Edgar Hoover, thus resulting in his demotion. He faked his own death several times and even ruined Simpson's chance to assassinate Adolf Hitler.
At one point, near the end of the war, the unit stormed a German castle. While searching through it, Private Burns found a collection of stolen paintings. While discussing what they should do with them, Burns mentions the idea of tontine. The unit agrees for their own personal reasons (Gumble wanting to buy his way into high society, Abe not wanting to end up "in one of them old folk's homes"). Everyone signs and shakes on this prospect.
Ox was the first member to die because of a hernia he got while taking the crate with the paintings out of the castle. Five more were killed in a parade float accident in 1979. After Asa Phelps died, only Burns and Simpson remained. Later, Simpson gave Burns a dishonorable discharge from the Hellfish for trying to kill him, and kept the paintings until the U.S. State Department took them away. In The Simpsons Game, The 'Flying Hellfish' was the army in 'Medal of Homer', and the insignia of the tontine was also seen in the Heaven level, on bungalows supposedly owned by the deceased members of the tontine, although those insignias are wearing halos and are called 'The Flying Heavenfish' possibly a reference to the fact that they are in Heaven.