Antoine is a seriously depressed singer with a rock band and the film begins with him walking out of that life, literally. Unskilled, and of a somewhat sullen disposition, he finds it difficult to find and hold down a job.
A woman at the job centre (agence d'intérim) suggests a job as a caretaker (guardien) at an apartment block, which comes with accommodation. He gets the job, not as a result of his interview, which is far from sparkling, but because the landlord's wife, Mathilde, played by Catherine Deneuve, takes a rather instant liking to him. Mathilde, as it turns out, is also depressed, worrying herself awake at nights because of a growing crack in one of the walls.
Antione and Mathilde are thus somewhat alike, and as he struggles with life, not helped by drinking and taking drugs, she becomes more and more obsessed with the state of the building. A particularly annoying tenant, Laurent, is continually bothering Antoine, while another young man, Stéphane, who also lives in one of the flats, presents problems by storing a number of probably stolen bicycles in the courtyard. Then Lev arrives, creating a whole new set of problems for Antoine.
Mathilde's worries about the cracks meanwhile escalates into a local campaign concerning the risks of subsidence, causing her husband to wonder whether she needs medical help.
Within this circle of flawed individuals one would not expect there to be a 'happy ever after' ending, but there is an awakening of sorts for Mathilde, which is due in no small measure to her friendship with Antoine.