When trying to imagine great screen couples of the future, few people would likely make the immediate jump to 49-year-old funnyman Steve Carell (2011's "Crazy, Stupid, Love.") and 27-year-old dramatic actress Keira Knightley (2010's "Never Let Me Go"). And yet here they are in "Seeking a Friend for the End of the World," a seriocomic love story with a relationship at its center that is so affecting it's a shame they keep getting interrupted by subplots and day players. Oh, yeah—and the looming apocalypse. First-time director Lorene Scafaria (who previously penned 2008's lovely "Nick and Norah's Infinite Playlist") has made an oddball hybrid, full of absurdist humor that suddenly doesn't seem so crazy under the tragic circumstances she sets up. Credit her and distributor Focus Features for sticking to their guns and allowing their premise to play itself out naturally. It may be one of the bleakest mainstream comedies ever released, but viewers willing to go with it will find a conclusion that isn't depressing so much as it is wondrously bittersweet.

Moments after learning that an asteroid named Matilda will indefinitely be colliding with earth in just three weeks' time, Dodge (Steve Carell) is abandoned by his longtime wife, who literally flees from the car they're sitting in. As the human race reacts in an endless assortment of ways to the end of days—some loot, some commit suicide, others feed their children booze while shooting up heroin and screwing anything that moves—Dodge finds himself comparatively numb. He continues to go to work, just as his dedicated housekeeper Elsa (Tonita Castro) keeps showing up to scrub away, afraid to face the truth of what's happening. With two weeks remaining, Dodge has a chance encounter with lively neighbor Penny (Keira Knightley). They've lived one door apart for years and have never spoken—a surprise all the more because of how well they hit it off. Fed up with her no-good boyfriend Owen (Adam Brody) and saddened about not getting to say good-bye to her parents—cell phones and airplane flights are no more—Penny decides to tag along when Dodge hits the road, on his way to see a high school flame he's just learned still loves him. In return, he promises that he might know someone who can fly her back to her family in England.

"Seeking a Friend for the End of the World" owes its blueprint and its unlikely star-crossed romance to a little-known gem of a picture, 1999's Don McKellar-directed "Last Night." It's not as taut or even quite as urgent as that earlier film, but it does carry many of the same themes and ideas. Tonally, writer-director Lorene Scafaria has a tricky balancing act to pull off. If it gets too silly, it will cease being believable. If it drowns too heavily in self-pity, all enjoyment will be sucked from its frames. While not always successful, Scafaria mostly gets things right. The early scenes are perhaps the most amusing from a laugh-aloud perspective. When a news anchor asks his field reporter about the traffic, her smiling reply says it all: "We're fucked, Bob!" Soon after, a party thrown by friends Warren (Rob Corddry) and Diane (Connie Britton) turns into a hedonistic free-for-all with no age limits required to partake.

The slapstick-laden bits level off when Dodge and Penny meet, the former feeling something inside again for the first time since his wife left and the latter just happy to have the company of a nice guy who's not a prick. Misadventures ensue as they must in the road movie genre—a scene where they stop at a restaurant and Dodge nearly gets much more than a birthday sing-along from the orgiastic servers is very funny—but there eventually become too many pit-stops along the way to Dodge's and Penny's realization that they love each other. A bit with a kindly truck driver (William Petersen) who has taken out a hit on his own life and another detour involving Penny's ex-boyfriend Speck (Derek Luke) slow the momentum down and detract from the lead pair's far more endearing, involving interplay. No matter what they do, underneath the humor and the eventual tears is a scary sense of impending finality. Even the comic parts are, in their own way, sobering.

With each new film he stars in, Steve Carell is proving how much more there is to him than just the comedian he started out as. He brings layers of pathos and vulnerability to Dodge that make a lasting impact, and he's matched in every way by Keira Knightley's heretofore untapped potential to slide effortlessly between naturalistic humor and heartbreak as Penny. The journey they take together isn't about their intended destinations, which lead nowhere special, but about the emotional connection they discover with each other when they least expect it. For all of its unevenness in between, this is what "Seeking a Friend for the End of the World" is actually about. Near the end, Penny bemoans that they didn't meet sooner, that they haven't been given enough time. "There never is," Dodge replies. The reverberating truth in those three little words is both simplistic and staggering. No matter what they're about to face, there's no other person they'd rather be with. Maybe that's enough.