With the aspiring singer Mila Malou (Suzy Delair), Wens follows clues to a seedy boarding house where he hopes to find the murderer.

Clouzot made several changes from the script including the characters Mila and Wens from his previous screenplay for Le dernier des six (1941).

Wen's mistress is the ditzy singer Mila Malou, who is seeking publicity to boost her struggling career and declares she will help him find the criminal.

Wens's first clue comes when a petty thief shows him a stack of theDurand calling cards that he found hidden in the attic of a boarding house at 21 Avenue Junot: the killer must be one of the tenants.

[12] A reviewer from Ciné-mondial praised the directing of Clouzot, stating that he "has put the finishing touches on a production that is dense, concise, mobile, varied, all in the service of a rich imagination".

[12] In the United States, a reviewer for The New York Times wrote that "The Murderer Lives at Number 21, despite a wandering script that fails to tie up many loose ends, is good fun for whodunnit fans".

Providing a 21st-century analysis, Bring The Noise UK reviewer Michael Dodd noted the "numerous brave little digs at the occupying Germans" present in the story.

He particularly singled out a scene in which a criminal has his hands raised, only to have one arm lowered by Inspector Wens so that he may light a match on the man's neck, thus making the villain look as though he is performing a Nazi salute.

"It is hard to believe that the strict German authorities missed the subtext of such a shot", he concluded "and the fact that he even dared to place it in the film at all is a testament to the character of Clouzot".