A CIA agent, referred to as the "Protagonist" (John David Washington), is drawn into a perilous extraction mission at a Kyiv opera house. The operation quickly escalates as Ukrainian forces have taken control, staging a hostage situation aimed at eliminating the operative. Under duress, the Protagonist receives orders to either extract the operative or eliminate him. As chaos unfolds, a mysterious masked soldier with a red trinket unexpectedly saves his life by executing the seemingly impossible act of "un-firing" a bullet through a gunman’s weapon.
However, the Protagonist's luck swiftly diminishes as he finds himself in the clutches of mercenaries, where he endures brutal torture and ingests cyanide. Awakening later, he realizes the cyanide was merely a test of loyalty; his team has perished, and the valuable artifact has vanished. In his search for answers, he is recruited by a shadowy organization named Tenet. His CIA superior, Fay (Martin Donovan), introduces him to the enigmatic scientist Barbara (Clémence Poesy), who enlightens him about bullets that exist with "inverted" entropy, implying they travel backward in time. Barbara’s research indicates these aberrant items may originate from a war yet to unfold, hinting at a dire future conspiracy unbeknownst to the present.
In his quest for truth, the Protagonist partners with Neil ([Robert Pattinson]), whom he meets through a CIA informant. Together, they trace the inverted bullets to Priya Singh ([Dimple Kapadia]), an arms dealer in Mumbai. They discover she plays a role within Tenet and her ammunition has been altered by the notorious Russian oligarch, Andrei Sator ([Kenneth Branagh]). Intriguingly, Priya mentions that she sold Sator standard cartridges but is baffled by how they’ve been inverted, suggesting that Sator serves as a bridge between their time and an unfathomable future.
Priya then connects the Protagonist with Sir Michael Crosby ([Michael Caine]), a British intelligence officer who provides further insight into Sator’s background. He recounts Sator's origins in a covert Soviet city, Stalsk-12, which was home to 200,000 people before a mysterious accident led to its abandonment. This location had been the site of underground tests, coinciding eerily with the day of the Kyiv siege.
Meanwhile, the estranged relationship between Sator and his wife, Kat (Elizabeth Debicki), complicates matters. She confides that Sator leveraged a forged Goya painting as blackmail against her, leaving her bound to him until the artwork is recovered. The Protagonist makes a bold offer to retrieve the painting to secure a meeting with Sator.
The Protagonist and Neil orchestrate a daring heist to steal the drawing from a secure storage facility in Oslo. Employing a strategy that involves crashing a plane into the vault's wall, they face off against two masked men who seem to be the same person existing in two different timelines. This marks their first encounter with the turnstile—a machine that can reverse the entropy of objects and individuals, one that Sator has, the purpose of which remains shrouded in mystery.
During an encounter on the Amalfi Coast in Italy, the Protagonist meets Sator and discovers the drawing is still safe, as Sator had it removed prior to the heist. Although Sator initially plots to eliminate the Protagonist, an unexpected turn of events allows the Protagonist to save Sator's life during an altercation instigated by Kat. In a twist of fate, Sator and the Protagonist form a tentative alliance to recover a case purported to contain plutonium-241, which was also the goal during the Kyiv operation.
The Protagonist continues to shadow Sator, observing the logistics behind his retrieval missions. His determination culminates in a violent confrontation in Tallinn, where the Protagonist and Neil manage to steal a case, discovering it actually contains the artifact lost in Kyiv. However, their victory is short-lived as an inverted Sator appears, holding Kat captive, thus leading to a reckless gambit.
In a culminating showdown, Sator's men and Tenet operatives collide in a chaotic battle, twisting through time and fate. Following a fierce struggle, the Protagonist and Kat are inadvertently drawn into one of Sator's machinations involving the turnstile. Desperate to escape, they are propelled back in time, facing off against their past selves while navigating the complex web of time inversion.
Further unraveling the plot reveals Sator's grand ambition to piece together an algorithm capable of globally reversing mankind's entropy—a catastrophic plan that could obliterate all of humanity's history. As Kat unveils that Sator is terminally ill, the stakes heighten. With Sator poised to unleash destruction during a planned vacation in Vietnam, the Protagonist and his team endeavor to thwart his plans.
In a suspenseful climax, a synchronized assault utilizes both inverted and non-inverted teams to confront Sator's defenses. Amid the chaos, Kat seizes the opportunity to eliminate Sator, just as the Protagonist secures the algorithm for Tenet.
In the aftermath, as the Protagonist begins to dismantle the algorithm, he realizes the intricate connections that ultimately led to him becoming the architect of Tenet. With layers of deception and loyalty intertwined, the Protagonist's journey has revealed both friend and foe, resulting in an epic tale that wrestles with the concepts of time, morality, and the consequences of our actions.
As the Protagonist reflects on the paradoxical nature of his mission, he contemplates the complex interplay of fate and free will while knowing the possibility of altering time hangs precariously in the balance.
