Meet Phineas L. MacGuire. Nickname: Mac. Goal: To be the best fourth-grade scientist ever. This is slightly revised to ridding Woodbrook Elementary School of mold. Mac has a lot to do.
He's growing penicillin mold for his good friend Aretha, and he's the campaign manager for his two best friends, Ben and Aretha, as they run for class president and vice president. He's working on the mold collection that will become the Mold Museum in the basement of the school. Then the best-laid plans begin to go awry, and Mac decides it's really hard to understand people sometimes.