In this National Treasure-like story, when Maggie disappears at a convent, Lisa goes undercover as a nun to solve a mystery involving a rare jewel.

The world eagerly awaits a total eclipse of the sun. People from England, Norway, Russia, Canada, and France sing the Credence Clearwater Revival song “Bad Moon Rising” in their native tongues as the moon’s shadow passes over the Earth. The Simpsons prepare to experience the rare event using some homemade eclipse viewers. Just before the eclipse, a fight breaks out between Homer and Bart, and Homer ends up accidentally crushing his viewer. Marge gives Homer her viewer, but can’t resist looking directly at the sun. Blinded, she’s rushed to Dr. Hibbert’s office, where bandages are placed over her eyes. Marge is told she must wear the bandages for two weeks, and avoid all stress. When Marge returns home, Homer and the kids are happy to wait on her. Chaos erupts when rats invade the kitchen. One of the rats grabs hold of Homer’s hair, and forces him to cook (as in the movie Ratatouille. Homer decides it’s time to buy some rat poison. He drives to the store, taking Maggie and Santa’s Little Helper with him. On the ride home Homer struggles with Maggie, who seems intent on playing with the poison bottle; and Santa’s Little Helper, who’s determined to take away Maggie’s toy bunny. Homer loses control of the car, and plummets off a bridge. The car lands on a tiny island, where he’s faced with a dilemma: How does he get off the island without leaving the baby alone with the poison or the dog alone with the baby?

Homer comes up with a plan to escape from the island. First he rows Maggie across the river, and leaves her at a convent’s doorstep. As he rows back to the island to retrieve the dog, a nun picks up the baby, and brings her inside the convent. Homer returns home, and tells the kids what happened. Lisa comes up with a plan: she’ll infiltrate the convent, and rescue her sister. Lisa shows up at the convent door, and claims to be a sinner who lost her way. Mother Superior allows Lisa to enter the convent. Before long Lisa is wearing a nun’s habit, and quizzing Mother Superior about the convent’s nursery. Lisa is mystified as to Maggie’s whereabouts. Lisa gets some help from a Groundskeeper Willie-like nun named Groundssister Willa. Willa urges Lisa to look heavenward for the answers she seeks. Lisa looks up, and sees a painting of a Spanish nun and the hand of God… which holds a giant, glowing jewel. Still puzzled, Lisa translates some Latin words on a nearby stained-glass window. The words urge the reader to seek God with heart and soul. Improvising, Lisa plays the song “Heart and Soul” on a nearby organ. Thanks to some elaborate mechanisms, the organ begins to change shape, revealing a stone wall and words written in two alternating styles of calligraphy. One of the messages instructs the reader to seek out Springfield’s biggest manmade ring.

Lisa concludes that the biggest “ring” in Springfield comes from the bell in the Springfield Bell Tower. Meanwhile, at home, Homer and Bart do their best to keep Marge from finding out that her beloved baby is missing. At the Springfield Bell Tower, Lisa discovers the Comic Book Guy and Principal Skinner dressed in hooded robes. They tell Lisa they belong to an ancient order of mystery buffs. Skinner tells Lisa about the legend of Sister Teresa of Avila, who had a vision of a jewel that would create an era of peace and harmony. A team of nuns was dispatched to find the gemstone, but when word of their quest reached a group of high-ranking Freemasons — led by Benjamin Franklin, George Washington, and King George the Third — the Freemasons staged a phony war of independence to cover up their mission. The nuns fled the violence, and settled in Springfield. According to legend, the gem will be revealed during the first moon after the solar eclipse. When Lisa finds out the bell in the bell tower is made of papier mâché, she realizes that the biggest “ring” in Springfield is the word “ring” in the Springfield sign, in the hills high above the city. Lisa, Comic Book Guy, and Skinner drive to the Springfield sign, where they’re confronted by Mr. Burns (a Freemason). As Lisa deciphers a clue on the Springfield sign, Burns instructs Skinner and Comic Book Guy to dig their own graves.

Lisa deciphers an anagram on the Springfield sign, and realizes that she is the gem St. Teresa referred to. According to legend the gem must be brought to the convent by sunrise. Mr. Burns flies Lisa and the others to the convent in his helicopter. When Lisa arrives, Mother Superior tells her that Maggie is the gem St. Teresa spoke of. When a ray of sunshine hits Maggie’s hair, peace and harmony shine on Springfield. Suddenly Marge shows up at the convent, and retrieves Maggie, ending the period of peace.