In 'The Murderer Lives at Number 21', a detective investigates a series of murders in a boarding house, where the residents and staff become suspects. The plot thickens as the detective uncovers shocking secrets and must race against time to catch the murderer before more lives are lost. With a mix of comedy and suspense, this French noir film keeps audiences on the edge of their seats.

Adrien Moulinet, a modest encashing agent in the Nortier bank, is also an inventor in his spare time. His latest revolutionary creation is the motorized roller skate. His problem is to be able to market them. Jules Petitpas, a jobless adman, is the right man for that.

Guy Carbonnel, a rich industrialist, has more eyes for rugby than for his wife Hélène. René Rivals, his long-time pal, takes advantage of it to court her but Guy is becoming suspicious so René has to find a way to deflect his friend's suspicion. He finally has an idea: he asks Lilian, a circus acrobat, to pose as his mistress. Lilian accepts and they embark on a series of eventful moments.

The struggles of a small business owner come to light in this film by director André Cayatte. The proprietor of a fabric shop, M. Baudu faces stiff competition when a department store moves in across the street, the first of its kind in 1860s Paris. On top of the stresses associated with the rival retailer, Baudu’s niece and two nephews take up residence with him after recently being orphaned. The niece, Denise Baudu, sees the writing on the wall for her uncle’s business so she takes a job as a shop girl with his competitor and despite her success the decision does not register well with the family.