Lisa is sent by Miss Hoover to pick up chalk, only to find the storeroom completely empty. She is suddenly pulled underground and finds herself confronted with what looks like giant hamsters and mutated men. After a fight, Lisa is captured and taken to a makeshift classroom alongside Janey and Ralph Wiggum (both also captured while checking the storeroom).
Lisa learns that the mutants were actually substitute teachers who fled underground with the hamsters when the students refused to listen to them, and their grotesque appearances are due to them drinking contaminated water dripping down from the Nuclear Power Plant. All the mutants want to do is teach students (as the hamsters are stuck on a Grade 1 reading level), but Lisa argues that the job of a teacher is not to imprison students, but to help them release their true talents. This seems to convince them and they are all allowed to go free. Sensing their sadness, Lisa then offers the mutant teachers a deal...
Later, Principal Skinner introduces one of the mutant teachers as Mrs. Krabappel's substitute for the day. The students are so amazed and eager to learn from him that it brings tears to his misshapen eye.
