“Transamerica,” the sophomore effort by filmmaker Duncan Tucker, is about as close as movies come to a character study. The plot here, a cross-country road trip, runs a distant second to the man-soon-to-be-woman who is both the subject and the driving force of the film. Bree (Felicity Huffman), as she has named herself, is a blend of female stereotypes and bad makeup, but Huffman achieves the remarkable feat of anticipating our own aversion and leveling it at herself, becoming her own harshest critic. At the same time as she attempts to fit society’s mold, Bree is convinced she should have been born a woman, a tragicomic paradox Huffman plays to anxious perfection. “Transamerica” shows us a social conservative trapped inside the wrong body, a control freak trying to deny the reality of her own anatomy. In short, when the film isn’t busy stoking its frail plot, it tenderly and hilariously gets at the messy imperfections of life.

Traveling the well-worn path of the American road movie, “Transamerica” doesn’t add much to its genre. A dishwasher at a Mexican restaurant and freelance telemarketer in her spare moments, Bree doesn’t have time for friends. The only thing keeping her going is a sex change operation scheduled for the coming week. So when she receives a phone call from Toby (Kevin Zegers), a juvenile delinquent claiming to be her son, Bree ignores him and hides the unwelcome information from her therapist (Elizabeth Pena). When the ruse doesn’t go as planned, she is forced to fly to New York and deal with her son before the therapist will give the go-ahead for the operation.

Once there, Bree bails out Toby, who has been hustling for money. Pretending to be a woman from a church group rather than facing Toby’s disgust at learning that “she is a he,” and that she is Toby’s genetic father, Bree frantically searches for a way out of her parental responsibility. She finally decides to drive Toby to Los Angeles where he can pursue his dreams of porn stardom. On the road the two have several adventures, dealing with such horrors as Bree’s Texan parents, until, somewhere along the way, Bree realizes that she is ready to accept the irreversible responsibility of becoming both a woman and a parent.

“Transamerica” is at its sharpest when dealing with Bree’s evolving sense of self. Instead of dishing out the expected pro-gay propaganda, Tucker (who both wrote and directed) shows Bree’s disgust with her own imperfect femininity. A quiet introvert, Bree wants to be a real woman, not a transsexual, and doesn’t feel any connection to the transgendered community. She is so repulsed by her masculine side that she refers to her onetime male name in the third person. Only through her interactions with Toby, the child of her pre-operational loins and in many ways the personification of her discarded masculinity, can she begin to accept both sides of herself.

Kevin Zegers does a fine job as Bree’s foil, exuding a careless natural sexuality which attracts men and women alike. But between predictable invasions of Bree’s comfort zone, a far more interesting character emerges. A budding siren, Toby craves sex, using it as a wall to block out real intimacy. This sort of male character has remained largely unexplored in cinema and, unfortunately, remains largely unexplored here. Ultimately, none of the supporting characters have the depth required to flesh out their roles — “Transamerica” is decidedly Bree’s movie.

Felicity Huffman plays the part with such neurotic warmth that her journey from an awkward hyper-feminine caricature to a real woman can only be described as fascinating. The journey from New York to L.A., however, is not quite as interesting. Several of the vignettes along the road are poorly constructed and too abruptly paced. They serve merely as unconvincing attempts to tease more plot out of the thin storyline. The film is better off during the in-between times, when Toby and Bree are alone on the open road. Teasing each other with rising affection, the two banter like father and son.

Bree leaves “Transamerica” in a very different place than where she started; her personal transformation intricately and feelingly acted out. However, a heavier storyline would have done this lighthearted film good.