Transamerica (15) <!-- none onestar twostar threestar fourstar fivestar -->

We can all name movies in which a man pretends to be a woman (Tootsie) and a woman pretends to be a man (Boys Don't Cry), or those body-swap comedies in which a boy inhabits a man (Big) and a girl impersonates a woman (Freaky Friday). And I'm still trying to blank Rob Schneider'sThe Hot Chick, in which the hirsute comedian pretended to be a teenage girl. But how about a movie in which a woman plays a man trying to change into a woman? Transamerica is surely a first and, thanks to a magnificent, unrepeatable lead performance, might well be the last.

Felicity Huffman, already acclaimed as one of TV's Desperate Housewives, plays Bree Osbourne, a gawky, anxious male-to-female transsexual who's about to achieve her heart's desire by having the operation that will make her "all woman". But then, a thunderbolt: Bree learns that back in college, when she was a guy named Stanley, a brief fling with a woman ("so tragically lesbian it didn't count") resulted in the birth of a boy, Toby, now a 17-year-old delinquent. Of all the rotten luck. Bree's therapist (Elizabeth Peña), kindly though she is, refuses to allow the surgery to go ahead until this familial tangle has been resolved.

So Bree goes to New York to bail Toby (Kevin Zegers) out, presenting herself as a Christian missionary - as if she needed another mask to complicate the imposture. If she's desperate to keep quiet about being a one-time bloke, she sure as hell doesn't want the boy finding out she's also his one-time father. Toby, a street hustler, doesn't seem the curious type in any case, his energies diverted by drugs, beer and an ambition to become a porn star in LA. This early part of Transamerica is fascinating and, in its intricate play of evasion and misassumption, weirdly comic.

What keeps the movie alive is Huffman's superbly modulated performance, a matter of body language as taut as a tightrope-walker's and eyes nervously alert to the ever-present possibility of being unmasked. Her prissy, fastidious way with words ("perchance", "glad tidings") and the pinky she crooks over her teacup are manifestations of an effort to seem genteel at all times, an exaggerated reflex that's obvious to everyone but her. Yet Huffman makes this act of hiding in plain sight something more than a turn; she expresses a terrible sadness in her yearning to be other than she is, and at the same time a dread of being ridiculed for just that reason. All that one could have wished for was a better movie in which to anchor it.

Writer-director Duncan Tucker takes father and son on a pretty but unexceptional trip across a land of gentle native Americans, postcard sunsets, larcenous punks and tentative generational bonding, which, considering the outlandish point at which it started, ends up very middle of the road. Fionnula Flanagan, as the control-freak mother from hell, offers a shrill trumpet-blast of hysteria that leaves a ringing in the ears. One also notes, with a sigh, the unsurprising way the rest of the picture falls into place: with so many conventional movies being veiled excuses for the last-reel declaration "I love you, Dad", one always hopes that the not-so-conventional ones might go a different route. While it's hard to begrudge Transamerica its conciliatory mood, there's just a sense that it lets itself off a pretty sharp hook. Too bad. Huffman's super-subtle performance remains the best reason to see it.