Josh Hartnett is a handsome young man in the Ben Affleck mold, whose open face is betrayed only by his beady little eyes. Not that he would hurt you; he doesn't have a mean bone in his body. But from "Pearl Harbor" to "Black Hawk Down" to the new "40 Days and 40 Nights" he simply shows the eager but vacant expression of a puppy. Puppies make nice pets but dull leading men.

Set in a San Francisco of the (very horny) mind, the premise, if you can call it that, of this soft-core-porn comedy is that young Matt Sullivan (Hartnett), after being thrown over by his great love Nicole (Vinessa Shaw), overcompensates by letting his libido run his life. Endless one-night (and maybe two-a-night) stands come to a head on Ash Wednesday, when he decides to do something for Lent: he will give up sex for those forty days and nights. But in a plot that is right out of a sitcom, and you can take your pick of sitcoms, he then meets Ms. Right, Erica Sutton (Shannyn Sossamon), who of course does not understand why he won't, uh, you know. And for no reason other than the film has another hour to run, Matt cannot bring himself to tell her his program. She is confused; he is in agony.

Meanwhile, back at Matt's dot.com company his coworkers start a pool on how many days Matt can last without sexual release. They also put it on the internet and take bets from around the world. Day after day those who've picked that date try to maneuver him into having sex, or at least masturbating. His own roommate Ryan (Paulo Costanzo) is no help either, forever bringing home new sex partners. Like "Friends" and "Seinfeld," no one in the cast has a life outside of this particular problem. But much of this is very wittily written and charmingly acted; it's the best part of the film. Sossamon, who was a vapid blank in the middle of "A Knight's Tale" last year, shows a bit more here as Erica; maybe she has a future in films.