Although the trailers probably framed this as an outright comedy, it is mostly a romantic drama with most of the ha ha comedic elements at the beginning of the movie, and the comedy throughout is a little more in the black comedy vein. I think the story is helped by being written and directed by a woman.
The film is almost exactly as the title indicates. Steve Carell plays Dodge, an insurance salesman who still goes to work although an Earth-ending meteor is set to hit in three weeks time. As the days tick by the movie shows us how different people react, some keep to their routine, such as the tv anchorman continuing to professionally report on the state of the world (Mark Moses), or Dodge's cleaning lady continuing to come every week. Others, like Dodge's friends (Rob Corddry, Connie Britton, Patton Oswalt, Melanie Lynskey), live it up like there is no tomorrow. And as we know the tomorrows are ending soon.
Dodge sees his downstairs neighbor Penny (Keira Knightley) crying when she breaks up with her boyfriend (Adam Brody). He lets her stay the night and later helps her escape during a riot. Fleeing in her car with an abandoned dog, each have a last desire--she's missed all the commercial planes flying out of the country so Dodge offers her to take her to a man he knows who has a plane, so that she can see her family one last time; he wants to track down a high school sweetheart after finding a mislaid letter where she says he was the love of her life. We can see that for him a lot of his desire is of the "what if" he had done this or that throughout his life, and yet he is still unmotivated to act until this last moment.
It becomes a roadtrip as they meet friends and strangers and we see how they too have reacted to this end. Penny has a military friend (Derek Luke) who is hunkered down in a bomb shelter. A man who gives them a ride has a death wish (William Petersen). They stop at a family restaurant chain where all the employees have taken matters in their own hands. They also hook up with Dodge's estranged father (Martin Sheen).