The night before Valentine's Day, Xander shows Buffy his V-Day gift for Cordy: a necklace with a heart-shaped pendant. After cracking a few jokes, Buffy assures Xander that Cordy will love his gift. She then slays a vampire which rises from its grave, and they continue on with their conversation.

The next morning, Harmony and her friends are shunning Cordy from their discussion groups. They mock Cordy for dating Xander, whom they perceive to be one of the biggest losers in Sunnydale. Elsewhere, Amy asks Willow and Buffy if they plan on attending the V-Day party at the Bronze later that night. Willow is going to see Oz play in the band, while Buffy plans on staying home. As everyone leaves and hands in their assignment, Xander notices that Amy does not have a paper to turn in. To his surprise, he watches as Amy stares at Miss Beakman, the teacher. Miss Beakman then reaches out her hand, as if she is taking Amy's paper which doesn't even exist. Amy then turns and leaves. Xander catches up with Buffy and Willow, telling them that he suspects Amy of messing around with witchcraft. Buffy and Willow remind him that it was Amy's mother, Catherine, who was the witch, and that Amy would never fool around with it. Giles spots Buffy and approaches her. Jenny Calendar sees them and walks over, but Giles brushes her off, saying that he and Buffy are busy at the moment. In the library, Giles warns Buffy that Angel has a long past of committing horrid acts on Valentine's Day. He refuses to tell Buffy any specifics, however, but he does suggest that she be careful. Meanwhile, in the factory, Spike and Angel give Drusilla their V-Day gifts. Spike gives her a necklace, while Angel gives her a freshly torn-out heart.

That night, Buffy receives a box of roses, which were left at her doorstep. Buffy opens the box and finds a card with only one word written on it: "Soon". At the Bronze, Xander sees Cordy and walks over to give her his gift. After admiring the necklace for a moment, Cordy breaks up with him. Xander is righteously furious, and it only gets worse for him the next day, as everyone in school seems to know that they broke up. Xander then notices Amy walking around, so he runs over and tells her that he knows she's practicing witchcraft. With the upper hand, Xander strikes a deal with Amy: help him cast a love spell on Cordy, and nobody will have to know that she's a witch. After arguing the idea for a while, Amy concedes and tells Xander that she needs a personal item of Cordelia's. Xander finds Cordy near her locker and demands that she return the necklace he gave her. Cordy says it's in her locker, but when she opens it, she hides behind the locker door so she can remove the necklace from her neck without Xander seeing. Meanwhile, Buffy walks into the library and shows Giles the "Soon" card. Buffy demands that Giles tell her everything she needs to know about Angel and Valentine's Day, so Giles sits her down and proceeds to read various anecdotes from his books.

In a darkened room, Amy and Xander perform the spell that should cause Cordy to fall in love with him. However, Xander soon finds out that the spell had no effect on Cordy. He walks into the library where Buffy is still listening to Giles read his books. Buffy consoles Xander, then suggests that they spend some time together that night. Buffy's suggestions get increasingly lurid, but before they get too close, Amy walks in and tells Xander that she needs to have a word with him. Outside in the hall, Amy tells Xander that the spell didn't work. Xander could care less now, seeing how his dreams of Buffy and himself are finally being fulfilled. However, Amy then suggests that they spend some time together later that night, without the spells and other witchcraft. Xander grows suspicious, and his fears are confirmed when another girl approaches and asks him to accompany her that night. Xander realizes that the spell actually worked. However, instead of affecting Cordy, it zapped every other girl in Sunnydale. Xander goes home to avoid everybody, only to find Willow waiting for him in his bed. Willow wants to him to take her right there in his bed, and when things get a little too steamy between them, Xander pushes her off and runs off.

The next day, every single girl in the school ogles Xander as he walks down the hall. He quickly enters the library and tells Giles everything that's going on. Before Giles can process the information, Jenny walks in and tells Giles that they need to talk. However, her attention quickly shifts to Xander, proving that the spell affected not only the girls, but every single female in town. Giles gets between the two and berates Xander for causing all this to happen. He then leaves Xander in the library, dragging Jenny along with him. After they leave, Xander tries to barricade the entrance to the library, but Buffy simply walks in, wearing nothing but a raincoat. She seductively asks Xander to undress her, but he can't, for he knows that what Buffy is doing is not out of attraction for him, but simply a result of the spell. Just then, Amy arrives and tells Buffy to leave. Instead, Buffy punches her to the floor. Angered, Amy gets up and casts a spell on Buffy, turning her into the small rat. Giles and Jenny return just then. While Xander and Giles try to catch the rat, Oz walks into the library and punches Xander for whatever he did to depress Willow. While Xander assures Oz that nothing happened, the rat escapes through the partly-opened library door. Realizing that the rat is gone, Giles commands Xander and Oz to find the rat while he tries to get Amy to reverse the spells. Giles soon determines that Amy must have made a mistake in the casting that caused Cordy to be protected by the spell instead of affected by it.

In the hallway, Xander notices a fight breaking out. To his horror, he sees a mob of girls beating on Cordy. The mob is angry at Cordy for breaking Xander's heart. Rushing in through the love-crazed girls, Xander pulls Cordy out of the pack and runs away with her. They run outside, only to find Willow and dozens of other girls. Willow is wielding an axe, for she would rather see Xander dead than with Cordy. While the mobs of girls battle each other, Xander and Cordy escape through the commotion and end up at Buffy's house. Joyce lets them in and, after sending Cordy upstairs to get some bandages, starts seducing Xander. Cordy is sickened by this, so she pushes Joyce outside. Joyce punches through the door window and struggles to unlock the door, so Xander and Cordy run upstairs and hide in Buffy's room. Thinking they're safe, Angel suddenly arrives and pulls Xander through the window and throws him to the ground below. Before Angel can kill him, Drusilla arrives and protects Xander. While Angel leaves, Dru offers immortal life to Xander. Before she can sink his teeth into his neck, the mob of girls finally track them down. They pull Xander away from Dru and pile up on top of Xander. Cordy runs out and pulls Xander out from the pile before quickly running back inside. All the girls chase them into the house. That is, everyone except Dru. Angel reminds her that she can't get in without an invitation. Xander and Cordy run into the basement and lock themselves in there. Meanwhile, Oz tracks the rat into the school basement. The rat is first menaced by a cat, then approaches a mouse trap. Upstairs, Giles and Amy perform the reversal spells. They first reverse the spell on Buffy, causing her to transform back to her human self. She quickly realizes that she is completely naked, so she asks Oz to find her some clothes. While Giles and Amy work on reversing the love spell, the mob of girls break open the basement door. Xander and Cordy run downstairs, but there's nowhere to go. Soon, all the girls gang up on top of them. Finally, Giles and Amy complete the reversal spell, and everybody returns to normal. All the girls look around, completely confused as to what's going on. Cordy makes up an excuse that they had just finished a scavenger hunt.

The next day, Buffy thanks Xander for not taking advantage of her the previous day. It seems that everybody retained their memory of what occurred during the run of the spell. Xander then bumps into Harmony, who is walking with Cordy and the rest of their friends. After Harmony mocks Xander, Cordy has had enough. She rips on Harmony and tells everybody that she can date whoever she wants to date. Cordy then walks over to Xander and takes his hand. Cordy is horrified by what she's just done, but Xander comforts her, and the two of them walk off, together again.