It's kind of wild to see this after I've been exposed (mostly second hand but still) to the world of *Korean melodramas for TV (wonder where they got these from) since this story, frankly, has many ingredients right from the start as our heroine Therese is orphaned (that's one) after a truck kills her parents (truck of doom!), leaving her to care for her sister viz their grandparents, who work at the family book store.
Meanwhile, there's a scummy amateur boxer thief who preys on a (slightly) older rich woman at the hotel where he hanging around (and gets in on tips of who to fleece from a young bellhop dips**t), and for a half hour in this story it doesn't seem like these two worlds will intertwine... until chance has him ride by the younger sister Denise and is taken with her and must have her and... well, it leads to her writing the letter with that title.
There is so much melodrama pudding mix in this movie that there's always the chance it'll get stuck to the proverbial bowl and not be able to get out again. What Melville has going for him is that Max, as played by Philippe Lemaire with this towering confidence, is the sort of sociopath who has somehow convinced himself that he can get by through life through his persistence and that a woman will never say no to him.
The thing is too, when we are first introduced to him it's in this scenario - as other men watch and kind of laugh and, you know what, we sort of do as well - and he doesn't seem like anything except an eager guy... but that is always the trap by and for these sexual predators, of which Max is on top of being a habitual liar and thief. There is a comment there for us as well as how these jerks tend to not only slide by through society but are rarely (if ever) punished criminally. What works so well is he is contrast with Therese, Juliette Greco (oh yeah from Cocteau's Orpheus), who is solid as a rock and is unwavering, but carrying a metric ton of pain and sadness and all this religious burden on her... and is not only a threat to Max, she is potentially his next conquest (or is it real love this time? Don't fall for it, audience!)
In other words, Melville has a batch of juicy pot-boiler stuff to play with here, not to mention (I think) his first time dipping his feet into a crime story involving robbery, which is Max's side hustle, and there's a perfect set piece where he sneaks into the one rich lady's hotel room at night, she catches him, and then he melts into her arms (just splendid, amazingly tawdry, no notes). He also has an ending that is sort of brilliant as this existential tragedy, even though it feels closer to a kind of poetic justice (good rule of thumb- don't run after a train if you are... on the complete other side of the station), and yet he never forgets to make Max and Therese painfully and awkwardly human.
I do think Denise is left by the wayside and after her letter and attempted suicide her character is under-written, and I would've liked more of the grandparents the couple of times we see them. But this is the exact length it needs to be for this sort of half Film Noir and half pulpy piece of melodrama, something that could be creeping up to the line of the earlier era of French poetic realism but there's a nastier undercurrent to the existential problems that Max brings on himself and everyone around him. This is all to say that not only is this underrated for the director, it just plays as an entertaining crime-soaked character study regardless of who directed it - and it's a great way to close out seeing his body of work on a very good one.
