Two ominous notes of music repeat at the beginning of “A House of Dynamite,” sounding like the score from “Jaws.”

Only this time the killer shark is a nuke. And it’s only 19 minutes away from an unsuspecting United States.

The “you’re all going to die!” alarm from “Hurt Locker” and “Zero Dark Thirty” director Kathryn Bigelow is the king of all downers.

Its deflating take is that America is woefully unprepared for a potential nuclear war at a precarious time in the world, and we’re sitting ducks less than a half hour from imminent, unprovoked annihilation.

“Dynamite” is an overcooked casserole of lofty “if”s.

What happens if essential equipment malfunctions? What if, at our most perilous crossroads, our nation’s leaders mute their strategic Zoom calls to instead phone their wives and kids? What if our top analysts have the day off? What if nobody has a clue what they’re doing?

That’s all plausible.

However, when you give a realistic scenario the aesthetic of a 20-year-old network TV show and a script that’s just “Armageddon” in glasses, its veneer of importance falls off.

The video-gamey dialogue often had me giggling, with its hackneyed attempts to quickly establish 20 characters we’ll otherwise learn nothing about.

“He says the prenup is ironclad!,” yells frantic Cathy from FEMA (Moses Ingram) on the phone.

“Did you see the ballgame last night?,” asks Tracy Letts’ Strategic Command General Brady as though he’s kicking off an improv scene at Upright Citizens Brigade.

It’s hard to imagine any president — fictional or otherwise — uttering “I listened to this podcast…” just two minutes before the complete eradication of a major American city.

Lax details like that strain credulity of a movie that fancies itself a giant blinking warning sign of what’s to come. Sure I felt existential dread from start to finish. Bigelow succeeds there. I also felt cringe.  

“Dynamite” unfolds in three parts, each an alternate perspective on the same breathless span of time — from the detection of a missile launched over the Pacific to it reaching its eventual destination.

Part One is set in an Alaska interceptor station and the White House Situation Room. Rebecca Ferguson is the MVP of the film as Captain Olivia Walker, a mom who tries hard to hold back tears while remaining in charge.

Part Two finds us in an underground Strategic Command Center in Nebraska. As its general, dry Letts is as easily eviscerating as ever. Scenes also take place in a field where a North Korea expert (Greta Lee) is watching the Battle of Gettysburg being reenacted. Rather on the nose.

And Part Three mostly follows an amiable, if foul-mouthed President of the United States, played by Idris Elba. There are shadows of George W. Bush on Sept. 11 when Elba’s commander-in-chief is whisked away from a youth basketball league and onto a helicopter to make life-and-death choices midair.

Meanwhile, Jared Harris’ Secretary of Defense sits in the Pentagon, knowing next to nothing about defense.

After the first section, Bigelow’s tightrope turns to Silly String. The core trajectory has been firmly established and nothing is going to change it. It’s not that kind of movie.

“Dynamite” actually — sometimes cheesily — is a lot like 1990s and aughts disaster flicks, except there is not much suspense as to whether or not the nuclear bomb will land, even though Bigelow casually tries to create some. “They don’t always go off,” a character says. Yeah, yeah, yeah. The mood is too dire and the message too clear for the crisis to be averted.

“Dynamite” is more about what happens if the initial strike isn’t prevented right away. Do we retaliate against or disarm North Korea, Russia and China even if we’re not sure who did it? Will acting fast save the US or lead to its total destruction?

Relevant and scary questions, all.

What a shame then how simply they’re undone by the many hard-to-believe characters saying mockable things. You’ll howl when Jason Clarke’s Situation Room admiral chirps, “Give me a shout if the world is going to end!”