Leela and Amy get involved in the sport of Butterfly Derby, which involves two pairs of scantily-clad women with butterfly wings fighting each other in midair. However, after proving to be bad at it, they decide to up their chances of winning by consuming Nectar, a steroid-esque substance. It turns them into hulking she-warriors who begin dominating their competition. Their rivals eventually shut them out of the Nectar market, which it turns out comes from Kif's homeworld.
While they obtain more of it, Fry winds up getting sprayed with butterfly pheromones, which makes him irresistable to Leela – and Amy as well. This causes them to quit Nectar cold turkey before their final fight, which ends up being a fight to the death. Fry ends up inadvertently saving the day when the pheromones change him into a butterfly, and he winds up trying to mate with the other competitors, saving Leela and Amy.