20x13 - The Simpsons

Lisa infiltrates a convent in order to steal back Maggie who was accidentally taken in by nuns. While in the convent, Lisa uncovers a series of cryptic clues leading to a hidden jewel in Springfield. With help from resident history buffs Principal Skinner and Comic Book Guy, Lisa sets off to find the coveted prize. But Springfield's Freemasons are also hot on the trail of the jewel, and Lisa must race against them to find the jewel and rescue Maggie.