Starring Seth Rogen and Katherine Heigl. Rated 18A.

In interviews, writer-director Judd Apatow has explained his movies—The 40-Year-Old Virgin and the new, equally funny venture Knocked Up—as being about “goofy guys who get pretty girls”. The filmmaker, also behind such smart TV shows as Freaks and Geeks and The Larry Sanders Show, is a master of the implausible. And here he pushes that simple concept almost to its breaking point.

In Knocked Up, Ben Stone (likable Virgin vet Seth Rogen) is a serious doofus and an unemployed, overweight illegal alien in the U.S. who gets really lucky one night. Not only does he go home with Alison Scott (Katherine Heigl of Grey’s Anatomy), the most gorgeous gal he has ever laid eyes or anything else on, they have the kind of sex that gets one of them pregnant. Oddly, even though Alison is as kind, clever, and talented as she is hot, he isn’t overjoyed to get this news. Even more oddly, she doesn’t take the first opportunity to plunk him on a fast train back to Loserville.

Not many women would feel compelled to include a guy they barely knew in their post-pregnancy plans. But much humour here is built around the absurd lengths Ben goes to in order to appear agreeable to eventualities that plainly freak him out.

Apatow also concentrates on the atmospherics of the un-couple’s separate worlds. Ben’s pals (including young comic staples Jonah Hill and Jay Baruchel) don’t have much ambition beyond the next bong—that and putting together an Internet guide to Hollywood nudity. Alison is a budding TV host still living with her older sister (Leslie Mann, married to Apatow in real life), who mercilessly picks on her depressed husband (Paul Rudd, in a subtly hilarious turn).

The movie doesn’t bother with the notion that an intelligent TV personality would have a few more options, even while preggers. There are also surprisingly few subplots for a movie that lasts more than two hours. What is on display, however, is very fertile stuff.