Phineas L. MacGuire—a.k.a. Mac—aims to be the best fourth-grade scientist ever. It's a tall order, but he's confident that he can achieve his goal, especially since Aretha has asked him to help her earn a Girl Scout badge by creating the mold that produces penicillin. How many fourth graders can say they've reproduced penicillin? None, as far as Mac knows. And who knows more about mold than Mac? But soon Mac is busy managing Ben's campaign for class president and dealing with his weird new babysitter. How can he focus on mold?