Homer's long-lost mother returns to the Simpson family. After some initial pleasantries, Bart and Lisa suspect that something's fishy about their new grandmother. It turns out that Mother Simpson is a fugitive from the law and has spent the last quarter century in hiding. In a flashback, we learn that when Homer was a small child, his mother fell in with a band of radical hippies who planted an antibiotic bomb in the germ warfare development lab of Springfield State University. Guarding the lab at the time was a young campus security guard named Clancy Wiggum. The lab was run by a younger (but by no means young) man named Monty Burns. Ever since the antibiotic bomb attack, Burns and Wiggum have been on the lookout for Mother Simpson. Realizing that it wasn't such a good idea to emerge from the underground, Mother Simpson bids her son and his family good-bye and returns to her secret life.