Giandomenico Fracchia is a shy and meek employee in a confectionery company, who endures bullying from his boss Dr. Orimbelli, his colleagues and even from his neighbors and cleaning lady.

However, the real Human Beast, who has been tailing Fracchia, breaks into his apartment, extorts his pass and sets up base in his house, ordering that the pair never be away from home at the same time.

While plotting a bank robbery, the Beast takes Fracchia's place at work, ultimately subduing Orimbelli and Miss Corvino.

Police commissioner Auricchio devises a plan to force the Beast out of hiding by leveraging his devotion to his elderly and overprotective mother, luring him to a clinic where she is being treated.

Memorables scenes are the gags between the Chief of Police Commissario Auricchio (Lino Banfi) and his subordinante Appuntato De Simone (Sandro Ghiani).