When a Valentine's Day rejection leaves Timmy the laughing stock of his class, he wishes that girls and boys lived on separate parts of the world. But soon Timmy realizes the consequences of his wish when he learns that both men and women feel a void inside, caused by a lack of love. And worst of all, because there's no love, Cupid is dying, and if Timmy doesn't put everything back to the way it was, Cupid will disappear forever along with love.
It's Valentine's Day and Timmy is in Fairy World with Cosmo and Wanda, watching a "military parade" of heart shaped vehicles and cherub soldiers led by Cupid. Timmy wants Trixie to be his Valentine, but when he asks Cupid who his special someone is, Cupid showed him a vision that it was to be Tootie, much to Timmy's disappointment. After an explosive love arrow sends Timmy and his fairies back to his home, he vows to prove Cupid wrong.
At school, many of the students are pairing together (including Elmer and a girl with a similar boil on her face), while Timmy searches for the right Valentine's gift for Trixie. Chester steps out from a locker and asks Timmy if he wanted to buy a beat up Valentine, but Timmy reminiscences what happened the last time Timmy tried to give Trixie a lame gift: he was quickly thrown off her property. Timmy screams as he remembers the moment he was launched in the air, while Chester slowly backs away from him.
With Wanda's help in the form of song, he puts together a musical number for Trixie, complete with a full stage set and dancers. However, Trixie rejects him yet again, and her catapult trap launches Timmy and his entire valentine act off her lawn once again. As Timmy frets that things could not get any more embarrassing for him, he looks up to see that every other girl in Dimmsdale witnessed his failure and was laughing at him. Tootie runs over to Timmy to stick up to him, declaring she loves him while giving him flowers, but this only causes the other girls to laugh at Timmy harder. He throws Tootie's gift at her, rudely shoves her away, and runs off away from her. A visibly upset Tootie gives chase.
Timmy evades Tootie by hiding behind a tree. Wanda tells him that he should simply let Tootie be his valentine, but Timmy refuses and blames Wanda for his Valentine plans going wrong. He wishes all the girls away from Dimmsdale. The city (and less apparently, the entire planet) is separated into "Himmsdale" and "Hersdale". All of the girls around Timmy disappear, including a distraught Tootie, a haughty Trixie surrounded by valentines gifts, Mrs. Turner, Vicky, Principal Waxelplax, and all other female characters, disappear from Dimmsdale and reappear in an open field. Within a few minutes, the women turn their side of the world into a beautiful paradise resembling Ancient Greece or Rome. On the other side of the wall, all of the women-related stores have been replaced with burger joints, athletic stores, and other male-centric things. Timmy and his friends enjoy the life of being a guy in song.
When the song is over, the separation of the two sexes is causing Cupid and his cherub troops to lose power. Their military style jets begin to fall from the sky. Even Cupid himself starts to shrivel up and become sicker and weaker from the "lack of love". As Cupid tries to figure out who is responsible for the mess on Earth, Timmy starts to notice that his father and other males are upset because something is missing in their life. On the other side of the wall, the women are experiencing the same pain of being unable to "fill the void" that was left when their loved one was separated from them. Timmy decides to set things right by tearing down the "Girl-In Wall" that his wish had created. Using a giant wished-up steak, he cracks the wall and it falls into pieces, bringing the two sides back together again.
The lack of trust and love between the males and females quickly escalates into full blown war. While Timmy fearfully watches the chaos unfold around him, Cupid figures out that Timmy is responsible for all the trouble, and uses a heart shaped portal to grab Timmy and pull him back into Fairy World. There, Cupid is almost corpse-like due to the lack of love magic. Cupid manages to provide Timmy with advanced weapons that operate like Cupid's arrows, making anyone they touch fall in love with the first person they see. Returning to Earth, Timmy uses the weapons to make the fighting males and females fall in love with each other again, including his own parents. As love begins to form everywhere, Cupid is returned to strength. Cupid and his cherub troops in jets fly over Dimmsdale and hit it with a giant love bomb.
With Dimmsdale returned to normal, Timmy ends up back in the park with his fairies. For his efforts, Timmy is given a love arrow by Cupid that will cause whoever it touches to fall in love with him. Before he can decide what to do with it, Trixie interrupts Timmy and tells him that she rejected him out of habit before realzing that his Valentine was the best, so she will be his Valentine because his was best, negating the need for the arrow. However, before he accepts her offer, Timmy overhears Tootie crying on a nearby park bench, and realizes the true value of love. Timmy tells Trixie that she does not deserve his Valentine's Day love, and that Tootie deserved it more. As Timmy walks off, the rejection causes Trixie to take an interest in Timmy (at least until the next episode), and she vows to leave him a thousand messages on his answering machine. Timmy walks over to Tootie on the park bench, and hits himself with the love arrow he was given earlier so he would fall in love with Tootie. She excitedly accepts his offer and they start kissing. Cosmo and Wanda see the growing intensity of Tootie's kissing from nearby, before Wanda decides to kiss Cosmo herself as well. The episode closes on Cupid saying "Isn't love a hoot?" and with Trixie leaving a message on Timmy's answering machine.