On the Sunnydale high football field, the Summers sisters discuss the complexities of love and relationships. Dawn wonders why on earth people spend so much time driving each other insane by getting involved in tempestuous relationships, but gets distracted when her gaze suddenly slo-mo's upon R.J., a handsome young buck of a football player. Dawn's irrational affection spurs her to make a total fool of herself, more than once, in front of R.J. and his pals. It's painful. When Buffy tries to comfort her devastated sister, Dawn won't have it: she lashes out at her sister, saying that no one so dysfunctional as Buffy would understand what she's going through. Ouch.
In school, Dawn overhears R.J.'s friend O'Donnell gloating about how he's replaced R.J. as starting quarterback in the upcoming football game. Dawn goes after O'Donnell to confront him, and in the heat of the moment shoves the kid down a flight of stairs. He ends up in the hospital, and Dawn ends up in Principal Wood's office to explain the "accident." Wood and Buffy believe Dawn when she lies that O'Donnell slipped, until the girl mentions how R.J. will now be replacing the injured boy as quarterback. After Dawn leaves the office, R.J. approaches her and asks if she'd want to meet up with him after football practice. Of course she jumps at the chance.
That night at the Bronze, Buffy, Xander and Willow see R.J. dancing with a sleazy girl dressed in a revealing outfit, and Buffy is relieved Dawn isn't there to witness the object of her affection getting down with a ho. That is, until the ho reveals herself to be none other than Dawn. When Buffy confronts her, Dawn leaves in a huff, declaring that Buffy's jealous because someone's finally paying her some attention. In the alley, Dawn gets attacked by R.J.'s ex-girlfriend and they end up in a screaming, hair-pulling catfight. Buffy breaks it up, but both girls have definitely lost it over the guy.
At school the next day, R.J. gets a talking-to from Wood over getting girls to do his homework for him, then gets another talking-to from Buffy over Dawn. But when R.J. slips his on letterman's jacket, Buffy's eyes glaze over and she's smitten. Later that evening, Buffy lies to Dawn that R.J. felt that Dawn came on too strong with him. For some reason, she's scheming to get the guy for herself.
Back at school, Buffy calls R.J. out of class and drags him into an empty classroom. Soon, she's got him in a liplock atop a desk, and he's not complaining. Dawn sees them, and stumbles out of the school crying. Xander finds her, and walks in on Buffy and her young stud still making out. He puts the kibosh on that fast.
It's a love spell, and Xander gets Willow to check out R.J.'s family online. While Buff and Dawnie have a yelling match over who R.J. loves more, the gang finds a listing for R.J.'s older brother, a bullying jock and former classmate of Xander's. The brother, a no-longer-bullying pizza delivery guy, says that the jacket was a simple hand-me-down from his dad, and he passed it on to R.J.
That jacket is mighty powerful, since when R.J. shows up at the Summers house to see Buffy, both Anya and Willow get bitten by the love bug. When the four women convene, they go all Witches of Eastwick as they shout about how they will each get R.J. to love them the most. Willow prepares to cast a spell on him (turning him into a girl), Anya sets out to rob a bank, Buffy gets ready to kill Wood so he'll leave R.J. alone, and Dawn, seeing no way to compete with her sister, lies down on some railroad tracks. Spike and Xander interrupt all their efforts, and Buffy rescues Dawn from the tracks, assuring her that no guy, even one as hot as R.J., is worth dying for.
Tentative allies Spike and Xander construct and execute a complex plan: they tackle R.J., rip his jacket off, and burn it. The spell is broken. The girls are back to normal, embarrassed about their crazy behavior. Except Anya, who seems to have some extra cash on her hands and offers to spring for ice cream.
Summary from Buffy.com