At first they hate each other, because, let's be realistic here, Summer is the absolute embodiment of everything Anna despises. She's such a cliché that it's sickening, all ditz and glitz, a spoiled rich kid with nothing else on her mind apart from appearances. And Anna knows that Summer is only into Seth because she thinks she can't have him anymore – it's so classic and predictable that Anna just has to roll her eyes at the whole thing.

Then all of a sudden they've got something in common. Anna doesn't count being into Seth as having had something in common – she still doesn't believe that Summer is really into Seth, Seth as a real person instead of just a toy that someone else has snapped up and she's suddenly decided she wants – but having shared, well, holiday-based-Seth-kissage, that definitely counts. They're equally mad at him, and suddenly hating each other doesn't seem to matter.

"It's, like, things like this that make me want to totally give up on boys," Summer announces one lunch time, and then she tells Anna to sleep over that night. Anna gets it, and then she thinks maybe she's got it wrong, but no, that night Summer's prancing around in sexy lingerie, and instead of telling Summer that skimpy black lacy things aren't the answer to everything, she thinks it's hot.

She doesn't have time to analyse events, because Summer kisses her and Anna kisses back. "Who needs Seth Cohen, right?" Summer asks, sliding her hand underneath Anna's tank top.

Right, thinks Anna, and for a while she thinks this could all be okay. Sure, the world's topsy-turvy because she's realising that not only is Summer attractive but also a pretty cool person, and a better one than she appears at first glance, but this could work out. They don't need Seth.

She believes this at the time, but the second he apologises to them, in his babbling but heartfelt way, she feels something inside her melt, and she's pretty sure the same thing's happening to Summer.

And she knows, without even having to think it through rationally, that this is the end of her friendship, of whatever it might have been, with Summer. They're back to being rivals, and it hits her that she's just as much of a cliché as Summer. She's one of those girls who places her relationship with guys above all else, who'll let a friendship fall to pieces for the sake of someone with a Y-chromosome.

She works on his comic book and draws a panel with Summer in it, then realises she's spent so long on Summer that the panel doesn't fit with the rest of them. Summer's picture isn't fun and funky and all bold lines and exaggerations, it's a carefully crafted portrait. Anna crumples up the page and starts over.

He likes it, and she thinks this could be it. She smiles and pretends not to notice that Summer is watching them. This is what she wants, right?

But then she walks in on him and Summer and she realises that it might not be what he wants. Wonder Woman. God. See, that's what sophisticated girls do, she thinks, girls like Summer who are beautiful and worldly, girls who have figured out that giving a guy a comic book places them in the good-friend category and that dressing up as a superhero moves them firmly into the hot-girl-worth-having-sex-with zone.

The trouble is that Seth doesn't know what he wants, and he'll have to decide soon. For all their sakes, she thinks, because if he can make a decision then maybe she won't have to. She watches Summer, the costume now hidden underneath her dress, walk away, and bites her lip. Then she calls out, "Hey". Summer turns around, and half-smiles at her.

Anna takes her hand and pulls her into an empty room, pressing her own lips against Summer's the second the door swings shut. "I couldn't let that outfit go to waste," she explains in between kisses, and the funny thing is, she means it. The more she gets to know Summer the more she gets why Seth is into her. And the more she sees Summer in revealing outfits, the more she wants her.

She doesn't need anyone to tell her that this is messed up. Her relationships in Pittsburgh were never this complicated – not that either of these relationships qualify as relationships, really, because Seth can't or won't choose, and she can't quantify what she has with Summer, whether it's a relationship or just sex or whether Summer doesn't even count sex between girls as real sex. She doesn't know.

When he invites her over, part of her knows deep down that she's lost him, and another part of her has hope. What does Summer have that she doesn't? If she didn't know Summer, if she didn't like Summer, if the answers to that question weren't ready to trip off her tongue, then perhaps she'd be a little more sure of herself.

But she's not entirely surprised when Seth rejects her. Still hurt, though. Still angry enough to turn up the music so loud that even she finds it makes her head pound, and angry when she thinks about what an idiot she's being now, listening to songs of rage just because of some guy, and what an idiot she's been all along, to think that Seth would ever choose her over Summer.

She's not prone to self-pity but she still finds herself glaring at her reflection and wondering why she can't look more like Summer. Why she can't be beautiful.

She has no plans for New Year's, so she winds up at the party her parents suggest she go to, and when Summer shows up with Marissa she tells herself that the little leap in her chest has everything to do with seeing familiar faces and nothing to do with how beautiful Summer looks.

"You've gotta kiss someone at midnight," Summer says later that night, when Marissa's talking to that Oliver guy. "How you spend New Year's Eve is how you're going to spend the next year. There are plenty of cute guys here – just pick one!"

The problem is, Anna has already realised, is that she doesn't just want to kiss some random cute guy. But apparently Summer does. Summer can go off with a guy based solely on the fact that he waves and smiles at cute girls from across the room. Summer can be completely oblivious to the fact that Anna isn't interested in that guy, because she has enough on her plate without adding a third person to her list of crushes.

And that's when she makes her own choice. She might have kissed Summer, if it had been midnight, if they had been standing there alone, if it had felt right. But Summer is flirting with a guy she's known for an hour, and Anna can either hang around and wait for life to start happening, or she can make it happen.

She leaves and arrives at the Cohen's house with a few minutes to spare before midnight. When she kisses Seth, she doesn't think about the fact that Summer has kissed them both, that in some way her presence still lingers on both of their mouths. Enough of the love triangle. Anna has made her decision.