In the small town of Springfield, the community faces an environmental catastrophe that stems from the pollution of the local lake. Despite clear warnings to keep the waterway clean, Homer Simpson, the bumbling patriarch of the Simpson family, dumps a silo filled with pig waste into the already toxic lake, pushing it over the edge. This act of environmental negligence results in the Environmental Protection Agency (EPA) taking drastic measures. The EPA, led by its head, Russ Cargill, decides to contain the disaster by placing a giant dome over Springfield, effectively quarantining the town and its residents from the outside world.

Trapped under the dome, the town's society begins to unravel, and the Simpson family finds themselves at the center of the town's anger once Homer's actions come to light. Facing ostracism from their community and the breakdown of social norms within the dome, the Simpsons escape the confines of Springfield to start a new life in Alaska. However, the guilt of his actions and the fate of the town weigh heavily on Homer, leading him to question his responsibilities towards his family and fellow townspeople. The family is then faced with a tough decision: whether to live a new life away from Springfield or to go back and save the town from the disaster that Homer inadvertently caused.