Heroes don't come much more unlikely than Leo Demidov, the investigator at the center of Tom Rob Smith's debut thriller "Child 44." Sure, plenty of detectives drink, or sleep around, or get a little rough with a suspect. Demidov is in a class all his own among unsympathetic heroes, though: As a fast-rising officer in the Soviet Union's state security force, the NKVD, Leo has arrested and sent to death scores, perhaps hundreds, of innocent people.

With a hero so monstrous, how can "Child 44" be so outstanding? Because much of Smith's drama revolves around the scales falling from Demidov's eyes. And because Demidov faces an opponent vastly more frightening than he is: not the serial killer who murders children with impunity, but rather the State, which turns against Leo and his investigation. It's the State's cold and ruthless suppression of the truth and pursuit of Leo that lend "Child 44" its urgency -- and give Leo the moral weight necessary to win our respect.

As the novel opens in 1953 Moscow, Leo is sent to the home of a family who insist that their young son, Arkady, has been murdered. Such an assertion is deeply troubling in a society in which, by official edict, "there is no crime," and it's Leo's job to remind the family that Arkady's death was an accident. "An officer must train his heart to be Cruel," says Feliks Dzerzhinsky, forefather of the NKVD. Leo, a true believer in communism's potential, has trained his heart well.

But when Leo's wife, Raisa, falls under suspicion as a traitor, Leo finds himself on the wrong side of a system in which, as he well knows, "The duty of an investigator was to scratch away at innocence until guilt was uncovered." His wife's innocence is irrelevant to an NKVD detective; "If no guilt was uncovered," Leo knows, "then they hadn't scratched deep enough."

As Leo and Raisa attempt to outrun the danger the investigation has put them in, Leo realizes that the death of Arkady was a murder -- one of a series of dozens of brutal child killings across the Soviet Union. Local police trained to deny crimes rather than solve them have declared the murders accidents, or pinned them on those who cannot defend themselves: the mentally deficient, enemies of the state, foreigners, even imaginary Nazis hiding out in the woods. As Leo, suspected of treason, is stripped of his title, his authority and his power, he commits his first legitimately treasonous act: He launches a surreptitious investigation of the child murders, convinced that a single man is responsible for all of them.

As Leo's authority erodes, his deductive skills grow. For starters, he can get people to talk to him more frankly without the authority of the NKVD behind him. But that's a mixed blessing; as his wife, Raisa, tells him after she reveals the enormous gulf in their marriage he'd never understood, "The problem with becoming powerless, as you are now, is that people start telling you the truth. You're not used to it."

Leo's dogged investigation allows Smith to paint a portrait of Stalin-era Russia that feels authentic, terrifying and bizarre. A mystery is in many ways the perfect vehicle for an author to explore an unfamiliar society. Forced by the author to poke around in the darkest corners of a nation's psyche, readers see how things really work and how people really live.

In "Child 44," a serial killer's grim work proves the perfect polarizing lens through which to view Soviet society during the era of Stalin; with the benefit of Tom Rob Smith's filter, we can see clearly how the toxic atmosphere of that time poisoned every single Russian man and woman, creating a culture of fear so pervasive that not to stop a killer of children would be the logical and proper thing to do.

"Our system is perfectly arranged," Leo tells a fellow officer, "to allow this man to kill as many times as he likes." Indeed, it's a dark joke in "Child 44" that any attempt to take action against the serial killer through official Soviet law-enforcement channels has dire consequences. In the actual case on which "Child 44" was based, many innocents and patsies were accused of murder by the State before the real killer was found.

All this might make "Child 44" seem like a slog or a history lesson, but it's not; it is, in fact, a tremendously exciting thriller. In a refreshing twist on the serial killer genre, our hero is in no danger from the murderer himself; instead, it's the State that puts our detective in peril, and that peril feels entirely convincing and frightening. A number of bravura set pieces -- including a gruesome escape from a Gulag-bound train, using only a dead man's tooth as a tool -- pepper the plot, suggesting that the inevitable "Child 44" film will be awfully exciting. (Ridley Scott has acquired the rights.)

"Child 44" has its flaws. Smith is not a born mystery writer, and the novel suffers when he's forced to write to his genre; we could do without a number of the scenes in which we meet a brand new character who we just know is going to be attacked in a few pages. Smith does his best to subvert such moments, but they still provoke some eye-rolling.

Leo is provided a Javert-like nemesis, Vasili, whose pursuit of our hero feels unmotivated and schematic. And Smith's final-act revelation of the connection between our killer and our hero, too, feels awfully melodramatic. But these are quibbles, really, about an inventive first novel that strips its hero of all he's believed in and forces him to build himself as a new and better man.