The first few moments of "Seeking a Friend for the End of the World" suggest this Earth-ending story is going to be a dark look at humanity when the entire population knows there's only three weeks left to live.

Then the script by director Lorene Scafaria settles into a sweeter tone that shifts this from a foreboding tale of death and destruction to a sweet -- albeit short -- love story.

Steve Carell, who turns in his best performance since "Dan in Real Life," plays Dodge, an average guy who faces the sad possibility he will die alone when an asteroid strikes Earth. Instead of waiting for the sky to fall, Dodge decides to use his last remaining days to reunite with his high school sweetheart.

He's joined by Penny (Keira Knightley), a free spirit hanging on to the cockeyed optimism that things might not turn out so dead. If the end does come, she's going to be alone because she missed the last flight home to England.

Credit must go to Scafaria for controlling Carell's performance. He, like Jim Carrey or Ben Stiller, can lapse into manic moments that tend to be more annoying than funny. Keeping Carell more centered works in harmony with the tone of this movie.

Carell can still be funny -- even when his scenes with Knightley go from chivalrous to cranky to cavalier.

There's a natural charm to Knightley that makes her characters come across as vulnerable and strong.

It's as if acting is so effortless for her that she's not so much playing a role but going through some slight metamorphosis to become the character.

There are some contrivances along the way, especially between Dodge and his estranged father (Martin Sheen).

But those moments are scattered enough that the movie never slips into being melancholy or melodramatic.

It also helps that Scafaria doesn't back away from black comedy, such as a scene with a suicidal truck driver (William Petersen).

The script could have been as heavy as an asteroid but Scafaria shows a knack for drifting between light and dark to create a very smooth landscape.

That's why the love story comes to the front rather than being weighed down by the heavy backdrop.

If an asteroid was headed toward Earth, watching "Seeking a Friend for the End of the World" wouldn't be a bad way to spend a couple of hours -- especially with a close friend.

MOVIE REVIEW

"Seeking a Friend for the End of the World," rated R for language, sexual content, drug use. Stars Steve Carrell, Keira Knightley, Patton Oswalt, Connie Britton, Gillian Jacobs. Directed by Lorene Scarfaria. Running time: 94 minutes. Grade: B