After another boring week in his life, Bart sees a commercial on television for a fun cruise and begs Homer and Marge for a family vacation. They tell him that the family is low on cash, so Bart chooses to sell everything he owns to fund the vacation himself. He comes up well short of the needed amount, so Marge and Lisa help by selling one valuable item apiece. Together the three have enough money to book the family into an economy cabin; once the cruise starts, though, a series of free upgrades places them in a deluxe cabin. They enjoy the wide range of activities onboard, but Bart's spirits sink when he hears the cruise director, Rowan Priddis, sing a song to the passengers telling them to enjoy the rest of the cruise while they can before they go back to their normal lives. Bart fears that the remainder of his life will be painfully boring and decides to make the vacation last forever.
Later, a huge onboard television screen displays an emergency message from a military officer, warning the crew and passengers about a deadly virus that has started to spread on the mainland. He says that all ships must remain at sea to ensure that humanity survives. The message is actually taken from a movie in the Simpson cabin's DVD library, set up by Bart to broadcast all over the ship. He also disables communications with the mainland by pouring hot fudge on a control panel. As the ship stays at sea, conditions deteriorate and the food supply starts to run out. Eventually, the cruise turns into a post-apocalyptic civilization, with gladiator arenas, marauders, capital punishment, and Priddis claiming kingship over the passengers.
Marge and Lisa discover Bart's deception and inform the passengers that the virus is a hoax. As punishment, the furious passengers maroon the Simpsons in Antarctica and head home. While hiking toward a research station for help, they notice a group of penguins; Lisa is fascinated by the chance to see them up close, but Bart thinks that their lives are boring and says that the ice slide they are riding down is just one isolated moment of fun. Lisa tells him that capturing and enjoying the best moments in life can make it fun, and Bart realizes she is right after Homer pushes him down the ice slide, with the whole family joining in. The final scene is a flash-forward to an elderly Bart in a retirement home, commenting on how much fun his life has been.