Lisa soon bonds with her paternal grandmother, but notices Mona runs inside the house when a police car drives by. Suspicious, Lisa shares her concerns with Bart, who raided Mona's purse and found several driver's licenses with different names. Marge and Homer wonder why Mona left her son and never returned for 26 years. The family confronts Mona, who reveals the truth about her past. In 1969, Mona joined a group of hippies to protest a germ warfare laboratory owned by Mr. Burns which was preparing to poison everyone in Springfield. The group detonated an "antibiotic bomb" inside the lab, killing all the germs. An angry Burns was trampled by the hippies while attempting to stop them. When Mona went back to help Burns, she was recognized as one of the perpetrators, forcing her to leave Homer and his father Abe and go into hiding.