And so began the fight career of two skimpy outfits and the feisty women who filled them. And though sometimes they got beaten, other times they got beaten badly. But just when it looked like they'd never win, they didn't.
–Bender, about Leela and Amy's Butterfly Derby career
Fry, Leela, Amy and Kif go on a double date at Elzar's restaurant. Kif gets nervous when Amy asks him to decide which water Elzar offers, but manages to make his choice. Amy then comments on the boots Leela was wearing, causing Fry to make cat-like noises. Leela asks Fry to knock it off, stating that girlfriends always talk trash to each other. When Leela comments about Amy's dress, Fry makes some more cat-like noises. Amy asks him to knock it off too, and states that if girlfriends talk politely to each other, there is a problem between them.
The next day, the Planet Express crew has to deliver big, heavy moonrocks to the Moon, which Professor Farnsworth claims that the Apollo astronauts apparently stole them and that they have to return them back to the Moon because they are part of their cultural heritage. Amy and Leela then begin to trash talk with each other, which again causes Fry to make the cat-like noises, ending with a moonrock falling on him. As the crew arrives on the Moon, they then deliver the now slightly lighter moonrocks to the Moon farmer, who actually wants the rocks to build a fence around his herd of Buggaloes. Leela asks him whether the moonrocks are really the Moon's cultural heritage, but the Moon farmer just brushes it off, exclaiming that they are a bunch of big city-dummies who got scammed into bringing back the rocks out of shame.
The farmer then informs them that if they want to see some real Moon culture they should come down to the Butterfly Derby on the same night. The crew then heads to the arena, where inside two girls are shown flying around, wearing a pair of butterfly wings on their arms and light, skimpy outfits. Leela comments that she wasn't expecting this as it is quite elegant and serene. As soon as she is finished an announcement is made, introducing the reigning champions of the Butterfly Derby, the Murderflies. They then begin fighting the two other girls, with Leela commenting that this is what she has expected. They then make quick work of the former two ladies. As their opponents lay defeated on the floor, the derby's host, Abner Doubledeal, tells the audience that there is a weekly Amateur Flapdown, where two volunteers from the audience can come to the stage and compete against The Murderflies for a winning prize of 50 bucks. Amy and Leela decide to volunteer. They both put up a good fight against the Murderflies, but are eventually defeated. After the fight, Abner decides to offer Amy and Leela a place in the Butterfly Derby, as they are constantly looking for 'fresh blood', to which Amy and Leela agree.
Bender is then heard narrating Amy and Leela's career in the Butterfly Derby. Amy and Leela decide to call it quits, since they have lost 15 matches in a row. But then they spot a trainer supplying 'Nectar' to two competitors of the Butterfly Derby, with one of them stating that it really gives a performance boost. They then approach the trainer, who agrees to sell some of her supply to them. Amy and Leela decides to give it a try, and instantly feel the effects.
As the crew is watching the news, which is showing Amy and Leela's rise of their Butterfly Derby career. The two then appear, having gained muscle mass, acting more aggresively and constantly drinking Nectar. Amy and Leela are then shown winning another round in the Butterfly Derby. Abner announces that they will face the Murderflies in their next match for the championship title, as the two teams exchange taunts with each other. After the match Amy and Leela approach the trainer for some more Nectar, but the trainer says that the Murderflies have bought all of her Nectar supply. Fry and Kif are complaining about how they cannot get their girlfriends attention anymore. Amy and Leela then walk in and wonder how they are going to get more Nectar. Professor Farnsworth then reveals the Nectar comes from Kif's home planet and suggest that they can go there to collect some more Nectar for themselves. Kif begins to get worried, but Fry asks him to play along, as he sees this as an opportunity to go on a romantic trip together with their girlfriends. Bender appears suddenly, all geared up to go on vacation.
As they land on Kif's home planet, butterflies are seen flying everywhere. The crew arrives at the Butterfly Reserve Park, where the Grand Butterfly Curatorshows up and warns them that it is now the butterfly mating season and asks them not to disturb the Male Butterfly, whom she describes as having orange colouring and having enourmous 'butterjunk'. The Curator then warns them about danger, but due to legal issues, cannot specify what the danger is (in case something else happens and she becomes liable). They split into teams, and while Amy and Leela manage to find the Nectar they are looking for, Bender, Kif and Fry stumble upon the male butterfly. Bender decides to smash it and pulls out a hammer. Kif tries to warn him, but Bender ignores him and walks closer to it. The male butterfly then flies up in the air, ready to shoot his pheromones in self defence. Fry cowers behind Bender, but as the male butterfly shoots his pheromones, Bender quickly hides his head causing the pheromones to strike Fry. Kif comments that the pheromone smells like "a skunk filled with feta cheese." Later, the crew arrives at Kif's parents' house, where they are invited to spend the night. Kif's parents then set some rules, asking the males and females to sleep in separate rooms. Kif's parents notice that Fry smells awful, with Fry admitting to it and deciding to take a shower. Amy then kisses Kif goodnight, but Leela finds herself strangely attracted to Fry and begins seducing him. Leela then asks Fry to leave his bedroom door unlocked, with Fry being confused at first but then getting what Leela meant
Leela is seen sneaking out of her bedroom. There is a knock on Fry's bedroom door, and Fry invites the person inside. They begin to make out under the covers, but then Leela then shows up, the person Fry is in bed with revealed to be Amy. Bender comes out from the bathroom and becomes surprised over the situation, dropping Fry's toothbrush and towel around him. The next morning, Leela and Amy begin arguing about last night's incident, while constantly making out with Fry at the same time. Back at the Planet Express the two girls continue to make out with Fry, which makes Professor Farnsworth scan him with a miniature version of his Smell-O-Scope. Professor Farnsworth finds out that Fry's sex appeal is a result of the pheromones that were sprayed on him by the male butterfly, and with the girls being overdosed on Nectar, they act like female butterflies wanting to mate with Fry. Professor Farnsworth explains that the only way for them to end this is to go cold turkey, with Amy and Leela agreeing to do so. At the same time Fry starts to feel "caterpillary", and starts inching on the floor like the insect.
Amy and Leela manage to kick their Nectar addiction, with Amy apologizing to Kif over her unruly behaviour. The ladies have lost all of the mass the Nectar gave them in their "flapping muscles" within a matter of days. Leela then notices that Fry has gone missing, when the camera pans to Fry, wrapping himself into a coccon on the Planet Express spaceship's left wing. Abner then shows up on the video screen, stating that their match against the Murderflies is about to begin. The crew quickly rushes to the Butterfly Derby arena.
At the match Leela thanks Amy for sticking together as they were recovering from their Nectar addiction, with Amy asking what's the worst that can happen (Amy: Hot lava?!?). Abner announces that since this is a championship match the floor will be replaced with lava instead. Since they have been weaned off the Nectar, Leela and Amy don't have the competitive edge as before, and the Murderflies make quick work of the two. Outside, Fry is about to emerge from his cocoon. The Murderflies are shown tearing off Amy and Leela's butterfly wings and are about to drop them into the lava, when Fry shows up in butterfly form. The Murderflies find themselves attracted to Fry (or as they put it, that awesome stank) and they begin to mate with him publicly. Abner is forced to call off the match, while Leela claims that she felt kinda left out of the action.
Back at Planet Express, Leela asks whether Fry can return back to his original state, with Professor Farnsworth stating that he couldn't, and that Fry's brain activity is now at zero. Suddenly Fry's butterfly form begins to crack, revealing that Fry is still intact in his human form, including clothes. The crew then cheers happily, as Fry remains dazed and continues acting strangely.