Paul

Director Greg Mottola (“Superbad,” “Adventureland”) is an expert at mixing raunch and sweetness. Simon Pegg and Nick Frost (“Shaun of the Dead,” “Hot Fuzz”) are experts at making funny but knowing and affectionate tributes to movie genres. Together, they’ve made an uneven but amiable road trip sci-fi comedy about an alien with sly references to everything from “Star Trek” and “2001” to “Alien” and “Battlestar Gallactica.” And, of course, “Close Encounters of the Third Kind” and “ET.”

It begins, as all pop-culture-obsessed stories should, at Comic-Con, the annual San Diego fanboy extravaganza. Two English fans, Graeme (Pegg) and Clive (Frost) begin their long-awaited first visit to America, starting at Comic-Con and continuing on a road trip to Area 51, Roswell, and other legendary UFO locations. They happily put an “Alien On Board” bumper sticker on their camper. But that doesn’t mean they are prepared to actually have a close encounter of their own.

And certainly Paul (stoner-ish voice of Seth Rogan) is not at all what they had in mind. He immediately reassures them that the business about the probes is just an urban legend. He’s been on Earth for quite a while, so he has had a chance not just to absorb a lot of American culture but to influence it as well (Steven Spielberg has a clever cameo). He thought he was a guest, but has learned he was a prisoner. Now a fed (Jason Bateman) and a pair of cops (“SNL’s” Bill Hader and Joe Lo Truglio) are after him and Graeme and Clive are in for an adventure beyond their wildest dreams, which were already pretty wild (as shown in their comic book).

They meet a variety of people along the way, including Jane Lynch as a sympathetic waitress and Kristin Wiig as Ruth, a fundamentalist Christian with a bad eye who wears a creationist t-shirt showing Jesus shooting Darwin. Paul and the Brits cause her to have massive cognitive dissonance, questioning everything she has ever believed. Wiig manages to make Ruth’s child-like delight in catching up on a lifetime of unused swearwords is sweetly innocent. Mottola keeps things going briskly with some surprising cameos as more people join the chase, including Ruth’s gun-totin’, Bible-thumpin’ father, some angry biker types, a woman whose life was transformed by a close encounter with Paul when he first landed, and the head of the shady government agency trying to capture Paul before he makes it to the mother ship. The crudity, drug humor, and attempted satire about fundamentalism fall flat most of the time, but the affectionate understanding of fanboys and their obsessions, the unpretentious sweetness of the friendship and budding romance, and a couple of plot surprises make this something to phone home about.