Ben (Mark Duplass) is a thirtysomething Seattleite whose party days are long behind him and who has a normal job, a house, and a loving wife (Alycia Delmore). One night his hippie-ish old pal Andrew (Joshua Leonard) shows up unannounced at 2 am, and before Ben knows it, Andrew's made some new "friends" in town and is dragging him to an all-night party. Drunk, and trying to look as with-it as the bisexual ladies they're hanging out with, Ben and Andrew announce that they, two thoroughly heterosexual men, are going to have sex with each other for an amateur porn video festival in town.

The rest of writer/director Lynn Shelton's funny and knowing film is about how Ben, Andrew, and Ben's wife deal with this declaration, once the guys decide to go through with it even after sobering up. Here I must stress that there is not one whiff of homoeroticism in this entire film: Ben and Andrew are kind of doughy and not at all attracted to each other. Shelton's after larger prey, and what she does is something akin to magic: she takes a farcical, over-the-top concept and treats it with absolute sincerity. There isn't a single disingenuous second in this film, partly due to the improvised-sounding dialogue and the fact that Shelton and her cast treat their characters with sympathy and respect. These people are smart, likable, and self-aware.

Shelton has made a chamber comedy (there are only five speaking parts in the whole film) that manages to cover a lot of ground: not just squeamishness over man-on-man sex - which is actually a very minor element - but male fears of aggressive female sexuality, fear of growing up and becoming boring, the shame of calling yourself an artist when you never finish anything, and most of all, the pressure to be as hip as humanly possible (which is especially a fact of life in Seattle - the "Humpfest" that they're entering is based on an actual event up there - where the white urban culture is rife with mockery, self-conscious irony, and an exhausting cooler-than-thou attitude. There's no way this story could take place in Los Angeles).

Humpday is like a (much) talkier and more ribald version of Kelly Reichardt's Old Joy, another recent film made by a woman about two men in the Pacific Northwest whose paths have diverged after what was once a close friendship. Old Joy is an interesting film, but it's nowhere near as entertaining or as thought-provoking as Humpday. Shelton and her cast have perfectly captured what it means to be liberal and in your thirties in 2009. If this describes you, then you need to see this film.