A Parisian couple decides to sell the unsanitary cellar in their building. A very ordinary, nondescript man, Mr Fonzic, shows up to buy it. Nothing unusual there, until the man moves into the space and makes it his permanent residence. The couple tries desperately to cancel the sale but to no avail. Worse, Mr. Fonzic becomes a threat to their family as he befriends their teen-age daughter on whom he seems to exert a perverted influence.
