When Cordelia dumps Xander on Valentine's Day he blackmails the witch Amy into casting a love spell. His goal is to punish Cordelia by breaking up with her. But the whole thing backfires when everyone except Cordelia falls in love with Xander. After a few strange situations Xander realises his mistake but going back to normal might not be that easy. To complicate things even more Buffy is turned into a rat by the lovesick Amy and the soulless Angel wants to show his affection for Buffy by killing one of her friends.
In the end all spells are reversed, Cordelia for once sees that being popular isn't the only thing in life and all of Buffy's friends live another day.
This episode has been one of my favorites right from the very first time I saw it. This is mostly due to the fact that I'm a huge fan of the character of Xander. But there are other things that are great about this episode as well. It is funny and we get to see we normally don't: A very sexy Buffy, a jealous Cordy and Jenny, Joyce and Willow hitting on Xander.
In my opinion this episode is important because it shows Xander making a mistake and taking responsibility by telling Giles that he messed up. And while the basic tone of the episode is relaxed and funny the love spell and its consequences are dead serious: Angel attacking Xander is real. And the anger of the rejected women makes them very dangerous.
I also think that it's important that Cordy and Xander are getting back together in the end. We never got a lot of insight into their relationship (except their fighting) anyway which is sad in my opinion. The way they treated each other always made me wonder if what they had was more than a fling. As far as I know we also never learnt whether they ever had sex.
But still: In this episode we get a glance at their feelings for each other. We never had a doubt about how Xander feels for Buffy and Willow. But here the audiences sees Xander risking something for Cordelia as well. And that's maybe the most important thing about the episode since it puts their relationship into perspective.