When Homer leaves Maggie on the doorstep of a convent to keep her safe as he rescues Santa's Little Helper from a sticky situation, the nuns at the convent take Maggie in. Lisa comes up with a plan to infiltrate the convent and rescue Maggie, and she befriends a Groundskeeper Willie-like nun named Groundssister Willa. After deciphering a series of cryptic messages at the convent, Lisa sets off to find Springfield's most coveted jewel along with help from an ancient order of mystery buffs led by the Comic Book Guy and Principal Skinner. But when Springfield's Freemasons, led by Mr. Burns, get wind of Lisa's quest, she must race to solve the mystery and find Maggie.