The most gruesome moment of Todd Robinson’s gruesome film “Lonely Hearts” doesn’t make it onto the screen. This is a bit of a surprise.

From its opening credits — which roll over stills of murder victims in 1940s-style clothes and Police Gazette-style photos, lying where they fell, in pools of their own blood — to its final, graphic depiction of execution by electrocution, the movie is not otherwise characterized by reticence.

Most memorable, perhaps, is what may turn out to be a classic splatter shot — in which a naked woman sitting astride a man is shot in the back of the head, so that her blood sprays over her partner.

There’s not much, in other words, that Mr. Robinson is likely to stick at. Yet at his film’s climax, the hero, Detective Elmer C. “Buster” Robinson (John Travolta), gingerly lifts the lid of a wooden crate that recently contained a child’s tricycle and peeps inside. Instead of seeing what he sees, we see him moan, close the lid, and stagger away to be sick.

Why the sudden access of delicacy? The film’s drawing back at this point suggests that, at some level, its director must still be capable of feeling shame for the prurience of his camera in seeking out images of violent death.

Like other recent stabs at mid-20th-century detective tales — last year’s “Black Dahlia” and “Hollywoodland” come to mind — “Lonely Hearts” covers its salacious itch with a period setting. The noir-era ambiance is to violence-porn what the jungles of Africa used to be to naked flesh: an excuse to show what would look outré in any other setting.

And, like those earlier films, “Lonely Hearts” gets an extra kick from being based on real events. The so-called “Lonely Hearts killers,” Raymond Fernandez and Martha Beck (played here by Jared Leto and Salma Hayek) met lonely war widows through personal ads. Raymond would romance them, promise them marriage, steal their life savings, and then murder them.

Martha was originally one of Raymond’s victims until he genuinely fell for her and allowed her to join in the scam, representing herself as his sister. By all accounts, including this one, she was the more vicious of the two. Together they may have killed as many as 20 women. They were tried, convicted, and executed at Sing Sing in 1951.

Mr. Leto and Ms. Hayek go to town with the lovers’ twisted psyches and their amour fou, showing (while they are allowed on screen) why and how their story has been made into movies at least twice before — as Leonard Kastle’s “The Honeymoon Killers,” in 1970, and in a Mexican version, Arturo Ripstein’s “Deep Crimson” (“Profundo Carmesí”), in 1996.

But Mr. Robinson’s excuse for revisiting the subject is that Buster Robinson, who, with partner Charles Hildebrandt (James Gandolfini), eventually caught the killers, was his grandfather. That may be why he spends so much of his time on the detective rather than his detection.

If the story of the killers’ pursuit and capture is an exciting one, you wouldn’t know it from this film. Buster’s detective work does get a certain amount of attention, but not nearly so much as his private life, which is shown in parallel with the criminal career of the killers.

Buster is a lonely heart, too. His late wife committed suicide for unknown reasons, leaving both Buster and their son, Eddie (Dan Byrd), heartbroken. Now Eddie is acting out — as they wouldn’t have said in the 1940s — and Buster is riding him pretty hard while trying to conceal an unofficial affair with Rene (Laura Dern), who works at the station.

It’s a pity to see so fine an actress as Ms. Dern with so little to do. There’s no real story in the part of the film devoted to the detective’s home life. Instead, there are what amount to repeated establishing shots of his pain and loneliness — which I guess have to stand in for the pain and loneliness of the killers’ widow-victims, since none of them is a fully-realized character.

Maybe it’s Mr. Robinson’s taste for gore that makes him want to keep us from knowing too much and therefore caring too much about the victims. Or maybe he’s trying to convey how desensitized Buster has become to others’ pain by dwelling so much on his own. Either way, he removed from the material most of what could have lent it moral and dramatic force.

All we’re left with is Buster’s sensitivity, the wise-cracking cross-talk between the cops, the sicko killers’ obsession with each other and, well, the blood. That’s not enough.

jbowman@nysun.com