As a long-time admirer of the beautiful Nathalie Baye, I found terribly poignant the moment when Commandant Caroline “Caro” Vaudieu of the Paris police — her character in Xavier Beauvois’s new film, “Le Petit Lieutenant” — crumples after receiving bad news as if she has been shot. But it is also a reminder of what has been left out of this naughty French movie, namely sex.

Sex implies vulnerability, and all of Caro’s energies are required to prevent hers from being exposed. When, upon her return to the Parisian Criminal Investigation Division, from which she has been exiled to a more bureaucratic job, she breaks into that familiar radiant smile, it is only to remind us of what we will be missing for the rest of the film.

A desexualized Nathalie Baye is like a non-violent Arnold Schwarzenegger. Why would a film throw away its principal asset?

In fact, there is a reason. “Le Petit Lieutenant,” which opens tonight at Lincoln Plaza Cinemas and the Angelika Film Center, is a film about blighted lives — lives with very little in the way of joy or love or hope. Several of Ms. Baye’s scenes take place at AA meetings, and she sits in them stony-faced, listening to others express emotion without, apparently, feeling any herself. We can understand intellectually what booze deprivation must feel like, but such obvious emotional deprivation in such a woman is like a slap in the face.

“Abstinence has not diminished my madness, far from it,” one of her fellow alcoholics says. But in Caro it has diminished everything — as has loneliness and an enduring grief for the loss of a young son many years before.

If he had lived, that son would have been about the same age as Antoine Derouère (Jalil Lespert), the eponymous young lieutenant. He has just arrived from the provinces by way of the police academy, and his naïve excitement at the prospect of being a detective in the big city enters Caro’s stagnant life and that of her cynical, hard-bitten colleagues, like a breath of fresh air.

Antoine had been drawn to police work, at first, by the movies. He couldn’t bear the thought of being stuck behind a desk. Caro warns him that much of the time of an undercover cop, even in Paris, is spent behind a desk doing paperwork, but she agrees with him that the odd gros-gros case — a serial killer, say — makes it all worthwhile, even if you only get two or three in a career.

When a drunken Pole (Arthur Smykiewicz) is murdered by a couple of Russians and they run out of leads, Antoine unself-consciously says, “Let’s hope they strike again.”

Instead of reproving him, Caro says wonderingly, “This really excites you, doesn’t it?” — perhaps remembering her own excitement about the job as she now remembers the taste for gin and nightlife that she has had to give up.

Antoine has left back in Normandy a young wife, Julie (Bérangère Allaux), who has no desire to follow him to Paris. The boredom of a policeman’s life in Le Havre has no appeal to him; the excitement of life in the city none for her. In the stalemate between them, though love survives, we see in embryo the emotional gridlock that seems to affect all the older characters. The only one of Antoine’s colleagues with any joie de vivre is a young Moroccan, Solo (Roschdy Zem), happily married with two small children to a French wife. But he is the victim of casual racism every day.

Mr. Beauvois spent research time with the Parisian police in an effort to acquire a feel for what, at crucial moments, he calls the “reality” of the contemporary policeman’s lot, and he tries to convey this by a certain flattening of the narrative arc. He doesn’t use the hand-held camera of traditional cinéma vérité, but he achieves a similar effect by making the exciting story of the hunt for the murderous Russians seem to emerge naturally and not too far from the dull routines of ordinary police work and ordinary police lives.

The problem is that this is familiar cinematic territory. David Letterman used to do a parody on his show called “Cop on the Edge” to make fun of what has become the movie cliché of police burnout. “That’s the French police for you,” says one of Antoine’s colleagues, “a lot of wine, a lot of suicides.”

Such self-consciousness about post-traumatic stress in those exposed to violence and death, and the various and unpalatable forms of self-medication it elicits (now also a staple of war movies), has the opposite effect to that which is intended. Instead of making the characters seem more “real,” it makes them seem less so. It’s really the emotionally cauterized older cops who have seen too many movies, not Antoine.

That Mr. Beauvois is dimly aware of this is suggested by his allowing Antoine to keep his enthusiasm to the end — albeit in a way that offers us little satisfaction — instead of becoming emotionally stunted and embittered.

At one point he describes to his father, on a visit home, his observation of an autopsy performed on the dead Pole. “I saw the heart and the lungs and the other organs laid out as if they were on a butcher’s slab,” he says. “And I thought of Mozart. How can that stuff compose such music?”

It’s all the reminder we need, perhaps, in this highly engaging film that “reality” is not only visceral and harsh and disgusting and disappointing, but also has room for the best of humanity.