Steve Carell and Keira Knightley, together as a couple who’ve fallen suddenly and madly in love? Surely the apocalypse is nigh.

It’s coming in three weeks, to be exact, in "Seeking a Friend for the End of the World," the feature directing debut from screenwriter Lorene Scafaria ("Nick & Norah’s Infinite Playlist").

An asteroid 70 miles wide is hurtling toward Earth, ensuring destruction and doom for the entire planet. Scafaria explores how people behave when the rules of polite society are stripped away, a premise that isn’t exactly novel — the world ended just last year, much more artfully, in Lars Von Trier’s "Melancholia" — but one that’s brimming with potential for absurdist, satirical comedy.

Within that setting, Carell and Knightley get thrown together. The pairing doesn’t make a whole lot of sense on paper — in the real world or on the big screen — but for the most part they have enough unexpected, opposites-attract likability and find themselves in enough strangely amusing situations to make the movie work. The mawkish third act, however, nearly destroys all that appeal.

Carell’s character, Dodge, is very much in the vein of the detached and depressed but wryly observant figures he’s played before: He’s an insurance agent whose wife takes off when news of the asteroid breaks. Knightley is his downstairs neighbor in the apartment building, Penny, a free-spirited, pot-smoking Brit with a penchant for classic vinyl records. She is your quintessential Manic Pixie Dream Girl.

Naturally, these two people need to go on a road trip.

All Dodge wants to do is track down his high school sweetheart, The One That Got Away, in hopes of rekindling the romance in his final days. (Clearly, she’s meant to represent everything he wanted out of life and never achieved.) Penny, meanwhile, is fresh off a bad breakup (from a ridiculously self-centered musician played by Adam Brody) and all she wants to do is get home to England to ride it out with her family. Dodge knows a guy with a plane who can help her.

Their journey (Scafaria also wrote the script) is buoyed by individual moments, and by some of the inspired casting that’s revealed at each stop along the way. Among the best scenes takes place at the beginning: an end-of-the-world party Dodge’s friend Warren (Rob Corddry) and his wife (Connie Britton) throw, where civilized, middle-aged people cavort in wild ways because … why not? Nothing matters anymore. Similarly, Dodge and Penny find themselves at a TGI Friday’s-style restaurant called Friendsy’s where everyone has clearly been doing Ecstasy and the possibility of indulgence and danger lurks in every fake-Tiffany-lamped corner.

But enjoyably odd moments like these give way to sentimentality by the end, as if Scafaria didn’t feel comfortable just letting her characters succumb to the inevitable. No, they have to "learn something." They have to find catharsis and redemption. They have to tidy things up before the end — and the film’s ending itself feels too tidy, as well.

The mess would have been more interesting — and more real.