On her night patrol through a cemetery, Xander shows Buffy the silver necklace he intends to give Cordelia for Valentine's Day. Buffy teases him for his girl while reassuring him of the gift. Xander is under no illusions as he is convinced that dating Cordelia is more perilous than slaying.
On Valentine's Day, Harmony and her followers insult Cordelia for going out with Xander, revealing to Cordelia for the first time that her relationship with Xander is not as secret as she thought. In class, Xander notices that Amy Madison avoided homework by hypnotizing the teacher. He offers to speak to her as Buffy and Willow remember Amy's "amateur psycho" mother. Giles appears and asks to speak with Buffy when Jenny Calendar sees him and tries to make up. Giles decides that Buffy is more important; they leave for the library.
There, he warns Buffy of Angelus' vicious history around St. Valentine's Day. Giles decides Buffy does not need to know the details and offers to patrol on her behalf. At the same time, Spike shows Drusilla a necklace that is immediately topped by Angelus' gift: a fresh heart that he found in a "quaint little shop girl".
Whiling that night away alongside her mom, Buffy receives a gift and an ominous note from Angelus. Meanwhile, Willow confesses to being a groupie of Oz and his band. Cordelia is again ignored by her clique at the Bronze, and she decides that her popularity is more important than her relationship. After sadly praising an increasingly smitten Xander, first for his clothes, then for the necklace, she breaks up with him. Xander is outraged at her timing.
Xander is humiliated the next morning when everyone at Sunnydale High is laughing at the way he was dumped and Buffy is too focused on Angelus to offer him any support. He spots Amy and blackmails her into casting a love spell so he can break up with a lovelorn Cordelia and "put her through the same hell." Amy agrees to do the spell, but needs a personal item of Cordelia's for it. In the library, Buffy convinces Giles to tell her of Angelus' atrocities.
Xander goes up to Cordelia demanding the necklace back, and she pretends to get it out of the locker, claiming she doesn't like it anyway, but doesn't let him know that she is still wearing it under her buttoned-up shirt. That night, Xander and Amy perform the spell.
The next morning, Xander talks to Cordelia, attempting to find out the effect of the spell but finds her unaffected. Xander enters the library as Giles is still giving Buffy information on Angelus. Xander is despondent and this time Buffy consoles him on what happened with Cordelia, and suggests that they get together to "comfort each other". Xander literally cannot believe his luck, especially when Buffy hints she might even give him a lap dance. The two then nearly kiss when interrupted by Amy, who wants to discuss the 'failure' of the spell. Xander tries to tell her that he's no longer interested, but Amy tells him they can just hang out... and Xander realizes that Amy and Buffy are both acting the same way towards him. Then a Harmony groupie wants to study with him at night. He runs home, only to find Willow hiding under his sheets wearing only a long shirt. She offers him her virginity and he flees.
The next day, all of the girls at Sunnydale High are pleased to see Xander in the hallways; the boys, less so. Cordelia also remains unaffected, the use of her necklace having protected her from the spell, rather than making her the target. Xander confesses to Giles, who is appalled at his foolishness. Jenny enters the library intent on conversing with Giles, but soon finds herself distracted by Xander's presence. When the Watcher looks for Amy to reverse the spell, he has to drag a moaning Ms. Calendar away. Xander blockades the door, only to find Buffy, wearing only a trench-coat (theoretically), willing to give him more than a heart attack. When he refuses to undress her because she is under the influence, she becomes enraged, thinking he is toying with her. Amy walks in seeking Xander too, so Buffy punches her hard in the face. Amy turns Buffy into a rat, sending her scurrying behind bookcases. Buffy-rat hurries out of the library when a confused Oz punches Xander for making Willow cry on the phone all night. Giles angrily sends Xander home, and sends Oz to find Buffy-rat.
On his way out, Xander sees Harmony and other girls attacking Cordelia, who are now angry that she had treated Xander so badly two nights earlier. He rescues her and takes her outside, outside only to find an axe-wielding Willow leading other Sunnydale females. Xander and Cordelia flee to Buffy's house, only to find Joyce pawing over him. They run up to Buffy's room but Angelus appears in Buffy's window and drags Xander out. Drusilla saves Xander from becoming Angelus' substitute present for Buffy and offers him immortality. She is distracted by Willow and her gang, and Cordelia pulls Xander back in the house. The mob breaks into the house with knives and sticks (except for Drusilla, who cannot enter, lacking an invitation). Cornered in the basement, Xander reveals to Cordelia that he meant the spell for her (leaving out that he meant it for revenge). She is touched.
Meanwhile, Giles had convinced Amy to reverse her spells. The Buffy-rat is switched back to human form just before eating the cheese on a mousetrap in the school basement, leaving Buffy stricken with a "slight case of nudity." Then, just as they are about to kill the young couple, the spell is lifted from the obsessed women in the Summers' basement. Thinking quickly, Cordy commends their scavenger hunt.
The next morning, all has returned to normal. The girls all retain their memory of the events, and while most appear to be dismissive (or choose to repress the memories), Willow's emotional history with Xander has left her furious, and she refuses to talk to him. Buffy comments that she was impressed by Xander's reaction to her seduction. Later, Harmony insults Xander again, and Cordelia finally faces up to her friends, professes her desire to date Xander, and walks away arm-in-arm with him.