Alfred Hitchcock polished his anecdote about how to become “The Master of Suspense” over the decades, refining his definition of “the ticking clock” thriller to the “bomb under the table” analogy he related for a TV interview very late in his career.

But Hitch figured this business of building unbearable tension early on. “Sabotage” became the textbook “ticking clock” thriller and the model for all that came after it way back in 1936.

A boy, dispatched to deliver film cans and a parcel, doesn’t realize there’s a bomb in the package. The audience knows the timer on the bomb is set for 1:45. The kid keeps getting delayed — by the friendly fruit merchant next door, by traffic, by a parade, hassled by the tram ticket collector. And the clock keeps ticking.

A monochromatic film both of its day and transcending that era in cinema, “Sabotage” is brisk, brief and ends rather abruptly. The romantic lead — not Hitchcock’s first choice — doesn’t bring much sizzle to the screen. And a couple of plot turns are old school meloddramatic.

But over 90 years later, that ticking time bomb sequence still has the power to alarm and even shock.

Hitchcock was on a roll by the mid-30s, already a “brand name” filmmaker, already doing his cameos (aptly enough, he’s glimpsed leaving a darkened cinema early on). However, his most prolific period would produce confusion in the canon.

His earlier 1936 film, based on a play that was inspired by the works of W. Somerset Maugham, was titled “Secret Agent.” “Sabotage” is based on Joseph Conrad’s oft-filmed novel “Secret Agent.” And in the most confusing addition to that “artist is someone who pounds the same nail over and over again” allegory, Hitchcock Americanized this plot and put it on the road across country in WWII for the similarly-titled “Saboteur,” the most delightful variation on that theme.

A foreigner (Oscar Homolka) runs a London cinema, a hard way to make ends meet in the middle of the Great Depression. He’s married a young American (Sylvia Sidney), who sells tickets, and taken in her much younger brother (Desmond Tester) in the bargain. Living behind the auditorium only saves them so much.

But Karl Verloc has a side gig. He’s taking money from a mysterious man whose overseas employer wants to strike “the fear of death” in the hearts of Londoners. The film opens as Verloc dumps sands into the works at a London power station, crashing the city into a blackout.

As the Brits laugh off this convenience, that act of sabotage plainly wasn’t serious enough. Another act is called for, this one widening the conspiracy. A bomb is to be set off during a Lord Mayor’s celebration at “the heart of the world,” Picadilly Circus.

Verloc must visit a bomb maker (William Dewhurst) and confer with other conspirators (Peter Bull plays one) if he expects this side hustle to finally pay off.

It’s just that the fruit-seller’s clerk next door (John Loder) is paying a lot of attention to him. And his wife. “There’s a mystery about me,” he teases after he’s stuck his nose in her business over refunds for tickets sold the night of the blackout. Mrs. Verloc is put out, and flattered by the attention.

That clerk in a white coat is actually Sgt. Ted Spencer of Scotland Yard. They’re keeping an eye on this menacing-looking movie-theater operator with the sinister name.

The people or foreign power pulling the strings are never identified. The script walks a tightrope between making them anarchists of novelist Conrad’s era or German or perhaps even Soviet puppetmasters.

Hitchcock demonstrates, early on, his grasp of what Hannah Arendt labeled “the banality of evil” when talking about ordinary Germans carrying out the Holocaust.

Bushy eyebrows and sketchy side-eyeing aside, Verloc is a little man, a cowardly cog in a machine who may protest participating in any act that will cause “loss of life.” But he does it.

He’s the fellow who entrusted the bomb to his wife’s teen sibling because the police are watching him too closely to “make the delivery” himself. His amorality is dissociative and narcissistic, prefiguring an American politician with heedless “burn it all down” to save himself proclivities.

Mrs. Verloc may be in the dark about everything her husband has going on, but we see the bombmaker’s family listening in as he discusses details of what he’s built and what it will be used for. That’s his side hustle. He runs a pet shop.

That celebrated “ticking clock” sequence is one of the most studied in all of cinema for a reason. The zooms in on the “parcel,” the masterful editing, the coded message and clockwork superimpositions, the syncopated score mimicking a clock’s ticking right down to the Jack Russell terrier he pets on that tram is all of a piece — building suspense, manipulating our fear of what might happen and to whom.

Generations of filmmakers, film scholars and film fans latched onto Hitchcock thanks to this and the many versions of such sequences the legendary filmmaker served up in the decades to come.

The 1930s “made” Hitchcock, sharpened his skills and established his reputation for tense thrillers with a dash of style, romance, edge and manipulations that could be obvious and sometimes downright fun.

“Secret Agent,” “Sabotage” and “The Lady Vanishes” weren’t Hitchcock’s very best work. But each of these key stepping stone hits set the table for future classics as this cinematic artist would identify the nails that he would, indeed, “pound” “over and over again” on the screen.

Rating: “Approved,” violence

Cast: Sylvia Sidney, Oscar Homolka, Desmond Tester, John Loder, Matthew Boulton, William Dewhurst and Peter Bull

Credits: Directed Alfred Hitchcock, scripted by Charles Bennett, Ian Hay and Helen Simpson, based on a novel by Joseph Conrad. A Gaumont/British Picture Corp. release on Tubi, Plex, The Roku Channel, HDNet, etc.

Running time: 1:16