The British born son of a diplomat, a Kings College dropout of Anglo-Indian heritage, William Henry Pratt was probably the most unusual looking and almost certainly the best educated lumberjack in all of British Columbia when he lied his way into a touring North American theater company and invented a more exotic stage name to go with his dark looks and sinister gaze.

Boris Karloff was born, albeit in the small time of touring theater. And if not for a fortuitous moment of unemployment as he passed through Hollywood at the end of World War I, theater is where he might have remained. Fifty years, 205 screen credits, a mid-career Broadway smash and late career revival as TV host and much-employed, beloved guest star later, he died at a ripe old age, a man feared in his youth, utterly beloved in his dotage.

“Boris Karloff: The Man Behind the Monster” is an adoring appreciation of a screen icon, one of the founding figures in the birth of The Horror Movie, a cultured man who made his good name scaring generations half to death.

Filmmakers Thomas Hamilton (director) and Ron MacCloskey (screenwriter) can’t have had much trouble rounding up scores of interview subjects to talk about Karloff and sing his praises. From directors Guillermo del Toro and Joe Dante and actors Ron Perlman and Christopher Plummer to film historians, critics and the great man’s daughter all sat down to wax rhapsodic about Karloff and his life’s work.

When “How the Grinch Stole Christmas” came along in the mid-1960s, Karloff was already that over-used word, “a legend,” the definitive Frankenstein’s monster, an icon in a genre he preferred to label “thriller” rather than the one that stuck — “horror.” But every Christmas, from now until Kingdom Come, there he is, plummy-voiced and menacing, warm and ruthless, a cartoon character with but one mission.

“I must stop Christmas from coming!”

The longest portion of “The Man Behind the Monster” is devoted, as one might expect, to his work on James Whale’s “Frankenstein,”: the 1931 masterpiece that most refer to as his “big break.” Karloff, who’d been an extra and day player in silent films for over a decade, preferred to think of 1930’s “The Criminal Code” as providing that.

“Monster” does a great job setting the table for Karloff’s sudden stardom, when he was so famous, overnight, that movie posters used just his last name as he won top billing in classics from “The Old House” and “The Black Cat” to “The Mummy,” “The Body Snatcher” and “Bedlam,” with “Black Sabbath,” “The Raven” and “Targets” as glorious 1960s curtain calls.

He had a stammer that he shook off and a lisp that he never did, and a deep, resonant voice that could be professorial and avuncular, or menace in its purest form.

We hear of the battle over the long-deleted murder of a child scene from “Frankenstein,” and how hard it was to convince the star that he was good enough for Broadway, playing a criminal whose plastic surgery made him “look like Boris Karloff” in “Arsenic & Old Lace.”

His checkered family history, and a hint of how he might have remained something of a disapproved-of outsider even after finding great wealth and fame, within a family of diplomats, is touched on.

His fragility in his later years is noted, underscored by the trooper he was even in his last appearances.

And academics and biographers renew our appreciation for the less-known classics on his resume, such as “West of Shanghai” and “the most racist mainstream film that Hollywood made in the ’30s,” “The Mask of Fu Manchu.”

His extensive TV work is given a going over too, hosting and sometimes starring in the anthology series “Thriller,” and this gem, a guest-bit on Dinah Shore’s Show which let him show off his singing chops.

I was tickled by the TV guest shots and most intrigued by the way historians describe the way “Frankenstein” and its makeup maestro, Jack Pierce, augmented Karloff’s chiseled, gaunt face into the way Frankenstein is thought of to this day.

Roger Corman and Peter Bogdanovich, who worked with Karloff, sing his praises as an actor and gentleman to work with. But it is fanboy and Oscar-winning filmmaker Del Toro (“Pan’s Labyrinth,” “The Shape of Water”) who sings the loudest.

“I had a religious conversion” on seeing the various “Frankenstein” movies Karloff made, “a Paul on the Road to Damascus moment…I saw my Messiah!”

Watching Del Toro’s films, we can believe it. Watching “The Man Behind the Monster,” we get it. “Horror” may predate Karloff, and even his contemporaries Lon Chaney Sr. and Bela Lugosi (who became a good friend). But if there’s a face that launched a genre, it was Karloff’s. Something to remember the next time you hear “Maybe Christmas (he thought) doesn’t come from a store. Maybe Christmas perhaps means a little bit more.”

Cast: Boris Karloff, Guillermo del Toro, Sara Karloff, Joe Dante, Ron Perlman, Leonard Maltin, Roger Corman, Peter Bogdanovich, Christopher Pummer and Kevin Brownlow.

Credits: Directed by Thomas Hamilton, scripted by Thomas Hamilton and Ron MacCloskey. A Shout! Factory/Abramorama release.

Running time: 1:39