A vast conspiracy battles militant truth tellers to keep The Biggest Secret of All as the world hurtles towards World War III in Steven Spielberg’s third crack at a “Close Encounters” story, “Disclosure Day.”

It’s predicated on the belief that the title of a Robert Wise film from the 1950s would still hold true in today’s conspiracy-riddled, scandalized coverup, with “wag the dog” geopolitical violence — “The Day the Earth Stood Still.”

The premise is that “We’ve been lied to for 79 years,” that the government and government entities have kept “the truth” “out there” and not owned-up to the presence of other life in the universe, that aliens have been visiting, watching and carelessly crashing on our little green planet on the regular since Roswell.

Spielberg brings his usual technical prowess and directorial sizzle to some impressive chases, 360 degree pans covering impressive sets and suspenseful escapes of the cliff-hanger variety. But what’s jolting about this big budget epic is how utterly conventional it all is.

And as it whelms and more than occasionally underwhelms, we’re forced to chew on the questions “Would this news interrupt TV coverage of a World War about to start” and “Would anything like this cover-up shock anyone gaping in awe at treason and naked theft in plain sight engineered by a covered-up international pedophile ring with Russian and Israeli fingerprints all over it?”

We’re living in an “abandon hope, all ye who enter here” era. Would “Oh, we’ve taken aliens hostage” even move the needle?

Josh O’Connor of “Challengers” and “Wake Up Dead Man” plays Daniel, a cyber-security insider who has skipped off with digital “archives” of the epic coverup, of Mother Ships encased in boiling thunderstorm clouds, crash sites, alien bodies and alien autopsies. He’s fleeing with his newish girlfriend Jane (Eve Hewson) from the head researcher and point man on all things aliens, Scanlon, given a sinister fanaticism by Colin Firth.

As Daniel and Jane fall into and out of mecenary hands, bubbly Kansas City weather bunny Maggie (Emily Blunt) is just trying to “weather shimmy” her way into an anchor’s chair before she ages out of that opportunity. And then something happens and she starts speaking in alien tongues during a live shot, collapsing with what looks like a seizure.

Some understand her clicking, clucking chatter, but most don’t. That “something” has changed her. She reads minds, empathizes herself out of work jams and traffic tickets. Her boyfriend (Wyatt Russell) is at a loss. And then “agents” show up at the hospital where she’s treated and her new extra-terrestrial instincts take over and they’re on the lam, too.

Nobody’s chances ot getting away in a surveillance state can be that good. But Scanlon has an ace in the hole. There’s this alien tech that allows him to reach out via ESP and hunt his prey, sometimes through the weak links in their lives — their romantic partners.

Meanwhile, all the leader of this “Get the truth out there” underground, Hugo (Colman Domingo) wants is to keep his people out of Scanlon’s clutches long enough to get them to meet, tell the world and make history.

The script, co-written by Spielberg and longtime collaborator David Koepp, shows us crop circles in the making, symbols explained and a vast parade of myths, “encounters” and lore of the UFO to UAP eras.

I half expected Richard Dreyfuss to make a cameo, if not Henry Thomas.

But this largely humorless thriller is generally as serious as a heart attack. And that’s a problem, as it doesn’t sell its premise or the long-held belief that civilization will be unmoored once we find that there are others out there, “superior beings” who aren’t Old Testament mythology.

“Animals” are vessels for alien interaction with the human race. Having the fakest looking CGI birds, deer and foxes since “Call of the Wild” is a real show-stopper in a $115 million thriller.

Suggesting UFO believers were “right” about “everything” is problematic, as their ranks are riddled with loons and charlatans and every year’s UFO documentary releases are riddled with provably false crap peddled by the credulous and assorted con artists.

The latter acts of “Disclosure Day” are a jumble of information overload as the picture flashes on bits of video “proof” that characters have to ask “Is that AI?”

Exactly.

Plot contrivances have desperate, high-stakes gamblers risking everything only to throw up their hands and an Alfa-Romeo hit by not one but TWO passing trains, with neither stopping to assess or whatever because we need to get our escapees further down that rail line to safety.

All that said, I went along with “Disclosure Day.” It’s suspenseful enough and within the broad realm f plausibility. Blunt, Firth and Domingo make us believe and stick around for the hopeful message we’re sure is coming because we need it.

But Spielberg’s best statement on this subject was the epic “Close Encounters of the Third Kind” of 49 years ago, with “E.T.” selling the same “You’ve gotta believe” ethos to kids. If an actual “Disclosure Day” is coming, chances are it won’t be as epochal as all that, as we’ve seen those movies, watched the “Signs” and sung along with “E.T.I.”

As with the covered-up Trumpstein files, we’ll believe it when we see it. Maybe it’ll even lead the newscast.

Rating: PG-13, violence, bloody images, profanity

Cast: Emily Blunt, Josh O’Connor, Colin Firth, Eve Hewson, Henry Lloyd-Hughes, Wyatt Russell and Colman Domingo

Credits: Directed by Steven Spielberg, scripted by David Koepp and Steven Spielberg. A Universal release.

Running time: 2:25