An unnamed CIA agent, the “Protagonist”, participates in an undercover operation at a Kyiv opera house. He is aided by a masked soldier with a distinctive trinket, who appears to “un-fire” a bullet through a hostile gunman. After seizing a strange artifact, the Protagonist is captured by mercenaries. He endures torture before consuming a cyanide pill. He awakens to learn the pill was a test of his loyalty; his team has been killed and the artifact lost.
The Protagonist is now employed by a secret organization called Tenet. A scientist shows him bullets with “inverted” entropy which allows them to move backwards through time. She believes they are manufactured in the future, and a weapon exists that can wipe out the past. Aided by a local contact, Neil, the Protagonist traces the bullet cartridges to arms dealer Priya Singh. He discovers she is a member of Tenet; her cartridges were purchased and time-inverted by Russian oligarch Andrei Sator.
The Protagonist approaches Sator’s estranged wife Kat, an art appraiser who unknowingly sold Sator a forged Goya drawing. Sator uses the drawing to blackmail her and keep her under his control. The Protagonist plots to steal the drawing with Neil from Freeport, a facility in Oslo Airport. There, they find a machine, a “turnstile”, and fend off two masked men. Priya explains that the turnstile can invert the entropy of objects and people, and that the masked men were the same person.
In the Amalfi Coast, Kat introduces the Protagonist to Sator, but learns that the drawing is intact. The three go boating and Kat attempts to drown Sator, but the Protagonist saves him. The Protagonist offers to help Sator retrieve a case he says contains Plutonium-241. In Tallinn, the Protagonist and Neil ambush an armoured convoy and steal the case, which contains the artifact lost in Kyiv. They are ambushed by an inverted Sator, who holds Kat hostage. The Protagonist gives Sator an empty case and he retreats. The Protagonist saves Kat, but is captured and taken to Sator’s warehouse.
The inverted Sator shoots Kat with an inverted round, while the normal Sator demands the location of the artifact; the Protagonist gives him false information. Tenet operatives led by Ives rescue the Protagonist while Sator escapes. The group take Kat through Sator’s turnstile to invert her, reversing the effect of the round. The Protagonist returns to the ambush site and chases Sator. His vehicle is overturned and Sator sets it on fire; the Protagonist is saved by Ives’ team again. Neil reveals that he is a member of Tenet.
The Protagonist and Neil travel back in time to Freeport. There, the Protagonist fights his past self and enters the turnstile to revert himself, followed by Neil and Kat. Priya explains that the artifacts are parts of an “algorithm”, capable of catastrophically inverting the planet that Sator is assembling.
Kat reveals that Sator is dying from pancreatic cancer. He will trigger the algorithm with a dead man’s switch, believing that the world should die with him. Kat believes Sator will kill himself during their vacation in Vietnam when they were last happy together. The Protagonist, Neil, Kat, and the Tenet forces invert back to that day so Kat can delay Sator’s death while Tenet secures the algorithm.
Tenet tracks the assembled algorithm to Sator’s hometown of Stalsk-12 in Northern Siberia. In a “temporal pincer movement”, red team troops move forward in time, while the blue team troops move backward. The Protagonist and Ives are aided by a masked corpse with a trinket on its backpack that returns to life. In Vietnam, Kat boards Sator’s yacht and kills him as the Protagonist and Ives secure the algorithm.
The Protagonist, Neil, and Ives break up the algorithm and part ways. The Protagonist notices a trinket on Neil’s rucksack. Neil reveals that he was recruited by the Protagonist years earlier, and this mission is the end of a long friendship. Priya attempts to have Kat assassinated, but is killed by the Protagonist, who has realised that he is the future mastermind behind Tenet.