Mildred's chances of passing the upcoming potion exam look slim; she is trying to study for it in the greenhouse with Mr. Blossom (even Blossom knows his potions better than her), while he feeds his plants tea leaves. Mildred and Maud are assigned to clean up the greenhouse, and while they're at it they find one marrow untouched and decide a growing potion will help Mr Blossom win the competition. Ethel and Drusilla overhear and get another brilliant idea.