Paul, about to marry a woman who makes him wear ugly sweaters and work for her dad (James Brolin) at a company that publishes hunting magazines, spends his bachelor party pretending not to be the groom for reasons too silly to get into here. He imbibes amply and awakens the next morning with beautiful tiki dancer Becky in bed beside him.
Becky explains that nothing actually happened. Paul assumes he'll never see her again. But two things make that impossible: Becky is a serial job hopper (thus likely to pop up in random professions all over town), and, of course, she's the bride's cousin. If all that seems a tad coincidental, it's really best not to get too analytical here; the screenwriters -- all four of them (including Meet the Parents scribe Greg Glienna, recycling his own bits) -- clearly didn't.
A Guy Thing doesn't lack for ideas. Rather, it has too many of them -- none well-conceived. (director Chris Koch's Snow Day had a similar problem.) The twists, most of them half-assed, just pile on. The running joke that gives the movie its title is the premise that every guy is willing to cover a lie for any other guy, even a stranger. (It's also a prerequisite in these films that no woman will ever believe a man caught in a potentially incriminating situation, even if his story is true.) It's an amusing joke, but it isn't used here to the degree it could have been.
The actors seem truly committed to the scattershot material, though. Shawn Hatosy, often cast as the best friend, is rarely this good. Lee is surprisingly convincing as a pushover given his history of playing unrestrained id for director Kevin Smith. And Stiles is at her best since she debuted in 10 Things I Hate About You. For once, we can understand why someone would find more than just her looks appealing.
Special mention should be made of SNL alumnus David Koechner, as Paul's balding stepdad with a John Wayne fetish. Opposite each other, James Brolin's tanned businessman and Koechner's beer-drinking lug bring to life a weird dichotomy of conservative archetypes -- it's as though we're watching a two-man struggle for the soul of the Republican party right there. If they'd made that the whole movie, they might have had a winner.