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 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.