A French man spies on a lovely younger woman across the way. When he's spotted by the woman shortly after being questioned by the police about a local murder, the man's simple life becomes more complicated.
Imagine a revisionist Fritz Lang's M (1931) that tried to be sympathetic to the child murderer, and you have an approximate, if somewhat exaggerated, portrait of watching this movie in 2024. Absolutely unearned ironic pathos in the ending revelation given that Monsieur Hire has been shown been violent towards women multiple times in the film, and it also grates that the film plays dangerously close to the stereotype of the fastidious/hypochondriac (violent) homosexual. What Roger Ebert generously calls an "erotomania" seems a lot like, at best, being an obvious sex pest.