Just as the world is due to end in an asteroid-obliterated inferno, the relationships of neighbours Dodge (Carell) and Penny (Knightley) implode with a whimper rather than a bang.
His wife silently walks out of their passionless marriage as they sit in the car listening to a DJ announce that his radio station will "be bringing you the countdown to the end of days along with all you classic rock favourites."
She's just kicked her feckless slacker of a boyfriend out after one too many let-downs and Dodge invites her into his apartment after he finds her weeping on the fire escape.
Together they hit on a companionable plan to fills the days until Armageddon - they'll head off on a road trip where he will pay a visit to his childhood sweetheart and she will hitch a lift aboard a plane to see her parents for one last time.
Writer-director Lorena Scafaria, who penned the amiable rom-com Nick and Norah's Infinite Playlist, repeats the gently witty tone but here marries it to something infinitely bleaker.
In fact, the first part plays out as a pitch black comedy with a suicidal Dodge still turning up for work at an insurance firm where a near hysterical boss points out job vacancies. Later, he reluctantly attends a party to distract thoughts from the end of the world where a group of 40-somethings mainline heroin and swap partners.
He glumly views the bizarre turn of pre-apocalyptic events with a resigned, quizzical melancholy but once on the road with Penny - avoiding the riots back in LA - he begins to emerge from his shell, attracted by her youthful kooky charm.
After a surreal visit to a diner called Friendsy's where everything appears to be on the menu - including the sexually-obliging staff - Dodge and Penny look to each other for emotional nourishment and the courage to face down the coming apocalypse.
The mash-up of skewed love and black humour shouldn't work...but it does thanks to the off-kilter chemistry between Carell and Knightley and a narrative that touchingly propels their burgeoning romance to the bitter end.