While the Simpsons attend a French Canadian circus called "Cirque de Purée", a wild blizzard hits Springfield, turning it into a snowsquall overnight. However, Springfield Elementary School stays open regardless of the storm, even though every other local school closes. Only some students, including the classmates of Bart and Lisa, show up while Skinner and Willie are the only faculty members at the school that day. To pass the time, Skinner plays his professed favourite film, a very low-budget, long-running, 1938 holiday movie called "The Christmas That Almost Wasn't, But Then Was". When the kids try to leave school, however, snow has piled up, trapping the kids inside with Skinner and Groundskeeper Willie, much to their horror.
Skinner tries to keep the children under control, forcing them to stay together in the cafeteria and eat apples, relish and (only if they behave well enough) mayonnaise and will not let them go home until the snow melts. After Nelson tries to escape, Skinner unearths his U.S. Army memorabilia and is reminded of the days when he commanded respect from his troops. Skinner tries to command respect from the students, and briefly succeeds after threatening to hang them in their clothes on hooks on the wall if they rebel. Bart tries to tunnel his way out, but Skinner stops him, and after Willie refuses to destroy the tunnel, Skinner tries to, but is stuck in the resulting cave-in. The students rebel and eventually tie Skinner up in a dodge ball bag. They then set about 'going nuts' such as racing trolleys in the school hallway, even burning numerous books in the school's library.
Meanwhile, Homer, with the help of Ned Flanders, sets out to rescue the children but crashes Flanders' car into a fire hydrant, which sprays water that freezes the car in place. Homer's repeated gunning of the accelerator causes carbon monoxide to flood the front of the car. Flanders and Homer get high from the fumes and wildly hallucinate. While the trapped children go wild with power, Skinner uses the school hamster, named Nibbles, to get a message to the outside world. Nibbles makes it to Ned and Homer, breaking the window and reviving them: they crash into a salt silo, melting the snow around the school and badly rusting the car but freeing the kids in the process.
Superintendent Chalmers appears and is ready to blame Skinner for the school's current poor state when Bart takes the place, amending things with Skinner. As he and Lisa leave with Homer and Ned, Homer starts hallucinating again making him see Lisa as a camel and Bart as a dancing girl. Homer tries to kiss Bart, who tries his best to escape, causing the car to lose control and crash, and camel Lisa, then says "Merry Christmas from the Simpsons"!