What would a Christopher Nolan movie be if there wasn’t someone onscreen to explain his labyrinthine plots? Audiences — at least the ones willing to brave going into a theater in our virus-plagued times — get their answer with “Tenet,” a maze-like action thriller that’s exciting in the moment, even if it’s gratuitously complicated.

We get our first dollop of extraneous plot exposition when a CIA Black Ops agent — played by John David Washington and referred to in the credits only as “Protagonist” — wakes up on a boat, after he thought he had taken a cyanide pill when captured. No, that was a test, says a shadowy fixer (Martin Donovan), who gives him an assignment to stop a plot to destroy the world. His only tip is a single word, “tenet,” which “will open the right doors, and some of the wrong ones, too.”

Protagonist soon learns that he’s meant to stop “something worse” than nuclear annihilation — and that something involves “inversion,” in which objects move backward in time. The pursuit of the main antagonist, a Russian arms dealer named Andrei Sator (played by Kenneth Branagh), will lead Protagonist to Mumbai, Oslo and London, and involve two other major players: Neil (Robert Pattinson), a fellow agent with a big secret, and Kat (Elizabeth Debicki), an art dealer and Sator’s emotionally abused wife.

At regular intervals, Nolan, who wrote and directed, drops chunks of dialogue to reveal hints about Sator’s whereabouts, or explain the ins and outs of “inversion,” or stress how close the characters are to armageddon. All this would be illuminating, if the audience could make it out through Nolan’s oppressively dense sound mix and the actors speaking in more accents than a Meryl Streep retrospective. (Besides Branagh doing a Russian accent, there’s Paris-born Clèmence Poesy, Indian star Dimple Kapadia, British actor Aaron Taylor-Johnson attempting an Australian twang, and an appearance by Nolan’s frequent collaborator, the legendary Michael Caine.)

After an hour or two of these indigestible breadcrumbs — during which we discover how a tumbling car un-crashes itself — “Tenet” reveals a couple of fundamental truths. One is that running the film in reverse isn’t quite the “Matrix”-level showstopper Nolan thinks it is. Another is that the quasi-scientific explanations are Nolan’s attempt at fooling the audience by making the script sound artificially brainy.

This is Nolan’s stock in trade: making the visually or narratively impossible sound plausible. It’s been his trademark through “Memento,” “The Prestige,” “Inception” and “Interstellar.” But anyone familiar with time-travel fiction knows the explanations are just window dressing, an intellectual stopgap to keep us from thinking too hard about the implausibility of it all. (On “Doctor Who,” for example, they call it “timey-wimey,” a catchall phrase for vocalizing the comically inexplicable.)

Still, Nolan throws a lot of eye-catching spectacle up on the big screen. The capstone — after the realistic jetliner crash into a building (because Nolan actually crashed a jetliner into a building) — is a climactic combat sequence, in which some buildings are reduced to rubble while other rubble reassembles into buildings.

“Tenet” also benefits from a sharp cast. Pattinson shows James Bond-level charm as the suave and slippery Neil. Debicki (“Widows,” “The Man From U.N.C.L.E.”) is both elegantly cool and emotionally vulnerable as she is dangled as bait to draw out Branagh’s sadistic Sator. And Washington reinforces his claim, first made in “BlacKkKlansman,” as one of the most dynamic leading men working today. Together, they all do a great job of making Nolan’s fiendishly incomprehensible thriller energetically engaging.

★★★
‘Tenet’
Christopher Nolan’s reality-stretching thriller is exciting, but don’t stop to try to figure it out.
Where • Theaters everywhere they’re open.
When • Opened Thursday, Sept. 3.
Rated • PG-13 for intense sequences of violence and action, some suggestive references and brief strong language.
Running time • 150 minutes.