Stretch Dude (Bart) and Clobber Girl (Lisa) must deal with not just protecting Springfield from crime and disaster, but also with Marge's request that they babysit Maggie. To make her less of a liability, they have Prof. Frink give Maggie superpowers by combining Bart's elasticity and Lisa's indestructability with her body, turning her into Bouncing Battle Baby.
Even with powers, however, Maggie still proves to be dead-weight to Bart and Lisa's heroics (such as how they can't do any world-saving while she's having her nap). When the Springfield Bank is attacked by the self-replicating supervillain Snakeskin (Snake), however, Maggie's bouncing powers allows her to smash through all his copies at once while Bart and Lisa capture the culprit. The day is saved once again.
Back at the Simpson home, Marge has asked our heroes to watch their pets.