The facts about the so-called Katyn Forest Massacre are now well established. It involved the mass execution of members of the Polish elite—military and civilian—by the Soviet secret police, or NKVD, in April and May of 1940. The victims, numbering more than 20,000, had been taken prisoner in the aftermath of the Soviet invasion of Poland in 1939, and their murder was specifically authorized by the Politburo and Stalin. The dead were buried in mass graves. In 1990, the Soviet Union, under Mikhail Gorbachev, formally admitted responsibility, and in 2010 the Russian Duma confirmed that finding. (One wonders whether the Putin regime of today would do likewise.)

In the closing days of World War II and the years following, however, there were no such public expressions of certitude. In April, 1943, the Nazi government announced the discovery of the graves, but the Soviets responded by claiming that the massacre had been committed by German forces in 1941. Despite evidence to the contrary, the British and American governments, deeply concerned about maintaining good relations with their ally, went along with Stalin’s denials and the shifting of responsibility to the Germans. In short, they covered up the truth in what they deemed a higher international interest.

Director Piotr Szkopiak, whose grandfather was among the Katyn victims, has now collaborated with Paul Szambowski on a screen adaptation of the latter’s play, which fashions a fictional narrative about an investigation of the massacre by a British reporter in 1946-47. It is based on an actual eyewitness report by Ivan Krivozertiev, a Belorussian peasant who had fled to the west with the retreating German army in late 1943 and wound up in Germany before being sent first to Italy and then England. There he was placed in a displaced persons’ camp under the name of Mikhail (or Michael) Loboda. In May, 1945, he had given a deposition to an investigator collecting material for the Nuremberg trials, in which he described seeing the massacre, but the matter was never brought up in the proceedings. He died in the countryside of Gloucestershire in 1947.

“The Last Witness” uses this material as a springboard for a thriller in which Loboda’s account becomes the basis for an inquiry into the event—and the subsequent cover-up by the British authorities—by a young journalist named Stephen Underwood (Alex Pettyfer, here with a prominent moustache and a perpetual gloomy look). Underwood’s brother John (Gwilym Lee) is serving at the camp where Loboda is brought as a sort of caretaker for the Poles, a duty he shares with Polish liaison Col. Janusz Pietrowski (Will Thorp). Stephen comes to the camp to visit John, but also the woman he loves—Jeanette Mitchell (Talulah Riley), the unhappy wife of Mason Mitchell (Henry Lloyd-Hughes), a member of the British security establishment that will apparently do whatever is necessary to suppress the truth about what really happened in the Katyn Forest.

Despite his initial hesitancy, Loboda (Robert Wieckiewicz) eventually tells Stephen his story, but Underwood’s editor (Michael Gambon, appearing in just a couple of scenes), who is tired of the young reporter’s insubordination, not only refuses to run the exposé but fires him. Even worse, Underwood is betrayed by someone he had trusted, and as a result Loboda dies, the result of apparent British collusion with Soviet agents. Still Stephen searches on doggedly, using help from a disillusioned worker in British military archives (Ian Midlane) to gain access to secret files on the massacre—most notably an actual 1943 report compiled by Owen O’Malley for Winston Churchill in 1943—that had concluded that the Soviets were almost certainly the guilty parties. Underwood is determined to see the investigation through, but his ability to get the truth out seems doomed.

The underlying historical material for Szkopiak’s film is fascinating, but the attempt to mold it into an old-fashioned cinematic thriller proves misguided, not only because the script fails to provide truly surprising twists and the sluggish directorial approach (along with Jo Dixon’s very deliberate editing) is enervating, but because Pettyfer’s one-note portrayal of Underwood is deadening. Much of the remaining cast is similarly afflicted with an air of slow-motion seriousness, though Thorp and Midlane bring some dignified world-weariness to their turns and Wieckiewicz manages hints of tragic depth in the haunted Loboda.

For a film that was probably done on a very restricted budget, “The Last Witness” is visually quite impressive, with Edward Ames’s glossy widescreen cinematography showing off Nick Turner’s elegant production design and Hilary Hughes’s costumes.

The Katyn Forest Massacre was a harrowing event even by the standards of inhuman brutality perpetrated in the course of World War II—and by Stalin during years of peacetime—and it deserves to be better known among the public. Andrzej Wajda’s 2007 film simply titled “Katyń” is more about its emotional aftermath than the event itself, but it is still more emotionally wrenching than Szkopiak’s stodgy reframing of it in conventional suspense terms. The writer-director’s personal commitment is undeniable, but the way in which he’s chosen to express it is disappointing.

Still, the historical reality is itself so powerful that—like 2016’s similarly dedicated but ultimately flawed “Anthropoid,” about the 1942 assassination of Reinhard Heydrich—you may consider it worth a look on that basis alone, especially since its message that all governments are capable of lying to their citizens in the name of expediency is always worth hearing.