Sometimes, it’s the cover-up, not the crime, the old political saying goes.

But sometimes, it’s the crime AND the cover-up.

That’s the thesis of “The Photographer of Mauthausen” (“El fotógrafo de Mauthausen”), a Catalan/Spanish story from the Holocaust. Because mass extermination of Jews, Gypsies, Slavs, homosexuals and others can be easily proven. Proving who did it requires eyewitnesses and hard evidence.

Francesc Boix (Mario Casas of “The 33” and “Witching & Bitching”) was one of several thousand Spanish exiles, refugees from the country’s bloody civil war, captured by the Germans when they overran France in 1940. Declared stateless by their fascist government, the Germans were free to make them among the first sent to concentration camps.

“We BUILT it,” Boix declares (in Spanish with English subtitles) in the voice-over narration, “it” being the Mauthausen camp in Austria. When we meet him, he is already a photographer, working for the camp officer (Richard van Weyden) in charge of photography, part of the meticulous record-keeping the Germans are famous for.

In the early days of the camp, they went to the trouble of sending condolence letters to the families of those murdered there, which is a bit of shock. But again, it was early in the war, mass extermination wasn’t necessarily on the horizon, and perhaps things were more Geneva Conventional in Austria at the time.

Boix and others, mostly his fellow communists, organized into “night and fog” resistance — altering the serial numbers on murdered prisoners’ in their photos, messing with that Nazi bookkeeping.

Boix is a trusty, and lies to a newly-arrived little boy (Adrià Salazar) who has been separated from his father, “to give him hope.” In a grim world of labor, privation and random, summary executions, it’s all any of them have to give.

Boix is dragged out to prisoner massacres in the forests, to document the deed, while his twisted boss, Ricken (van Weyden), concerns himself with the odd-looking; dwarves, one-legged prisoners, etc.

Stumbling across negatives in the dark room, Boix notices that the bookkeepers are keeping everything. Listening in to the camp radio his fellow inmates have conjured up, he hears the tide of the war turning in the East. What will their monstrous, murderous and “victorious” captors do when they realize the war is lost?

Saving that film becomes a mission.

Director Mar Taragona, better known a producer of Spanish films (“The Orphanage”), and the script immerse us in the nuts and bolts of mass extermination — an early model of the vans used to gas victims is assessed. And the boilerplate characters and ingredients of concentration camp/POW camp movies can’t help but make appearances.

The sadism and utter disregard for life, especially by the SS kommandant (Stefan Weinert, properly monstrous), the plucky inmates figuring out ways to fool “the very best people” put in charge of such camps, the ingenious “systems” devised for hiding contraband or planning escapes. There’s even an amusing Spanish stage revue they put on to cover an escape attempt (the Germans don’t realize the Spaniards are singing about Russians and Americans on their way to liberate them).

“The Photographer” reminds us how hard it is to say something new on this subject, to avoid third act melodrama, even when the tale is essentially true. A lot of luck and chance were involved in deciding who lived and who died.

More attention should probably have been paid the photographs, stark images of murder, victims lying on snow, dangling from fences, etc. Several scenes take on a far-fetched air, a dubious dinner party interrupted by teaching a child how to shoot his first “sub human.”

But “The Photographer of Mauthausen” does boast of a novel demographic — Spanish victims, communists — and Casas makes a sturdy lead, playing not just someone who endured a camp, not just someone who could bear witness, but a man who made saving the photographic evidence something he was willing to die for.

MPAA Rating: TV-MA, graphic violence, sexual situation, nudity

Cast: Mario Casas, Richard van Weyden, Alain Hernández, Adrià Salazar and Stefan Weinert

Credits: Directed by Mar Taragona, script by Roger Danès, Alfred Pérez Fargas. A Netflix release.

Running time: 1:50