Friendship isn't rocket science. It's much harder.

Oh, yeah, it's easy to say just be loyal and true and that makes you a good friend. But suppose the other person does something that really irks you, like chew gum or vote Democratic? Suppose the other person laughs weirdly or thinks you can wear cheap shoes with an expensive suit, while you favor expensive shoes and cheap suits? Suppose the friendship is based on a subtle hierarchy, or one person's adoration without the other's reciprocation? Suppose he's just luckier or you're just luckier. Suppose you make more or you've always made more, and suddenly she's making more. Suppose she doesn't like the new boyfriend. Suppose there's jealousy, envy, bitterness, grudges, betrayals? It's way overrated, friendship.

And that thorniness, that dark underbelly of it, is the gist of the acerbic British import "Me Without You," which watches as a fab friendship comes unglued over the years. Set in suburban England, it follows as Marina and Holly come apart and, as the old song has it, breaking up is hard to do.

Marina (Anna Friel) is the pretty one. She's the dominant one. The boys always come on to her first, and then they gradually notice Holly (Michelle Williams), who lacks Marina's take-your-breath-away looks, her insouciance, her assurance at the game. So Holly is the eternal sidekick over the years.

The director, Sandra Goldbacher, working in an autobiographical mode (she was the real-life Holly, apparently), does a trim job with the cavalcade aspects of the movie, shifting effortlessly via careful clothing and music codes from '70s to '80s. And she is brutally without illusions on the topic of young womanhood in England, presumably because she is still a young woman in England.

The best sequence takes the two pals to a provincial university, where the lurking big dog is a visiting American professor played by the world's second-largest chin, Kyle MacLachlan. (Answer to obvious question: Bruce Campbell. Imagine those two in the same movie!) He's a superb actor, and he's superb in this role, which is to say I wanted to punch his teeth out and watch him suck broken Chiclets in the gutter for a while.

For a change, this monster of narcissism is attracted first to Holly. Why, how astonishing. Holly, it turns out, is very bright, and she has a natural critic's mind: she cares about writers and ideas, she's extremely insightful, and soon enough those little conferences over her essays have turned to petting sessions, and then we're rounding the bases and stealing home.

At the same time, for reasons that are mysterious to Marina but clear as day to us, the beautiful Marina feels compelled to get in on the fun. It's not that she's articulate enough to express herself, but sooner rather than later her little flirtship with the big creep has moved into the way of all flesh. The idea -- she isn't insightful -- that she considers it her right to have first pick of all men never occurs to her. That she is betraying a friend never lights up in her dark brain. She just has this urge to restore what she feels is the natural order of things.

But almost always, a key issue of friendship is forgiveness, and once the two have moved on, they naturally reconnect: There's too much water under the bridge to do otherwise.

Or is there?

There comes a time when Holly has to get what amounts to a divorce from Marina, and it's as painful as a legal divorce. To be the woman she wants to be, she can't be a sidekick anymore. This is the kind of small, real emotional issue that almost never makes it into movies. But the truth is, of course, that friendship matters to those of us who still claim membership in the human race, and Goldbacher's merciless autopsy on it is both illuminating and dispiriting.

Me Without You (107 minutes, at Cineplex Odeon Dupont Circle) is rated R for sexuality and profanity.