What is the algorithm?
To understand anything of what’s happening at the end of Tenet, you have to understand the algorithm. The algorithm is the scientific formula that allows time to flow backwards, or become ‘inverted’. It will be developed by scientists in the future, and – fearing its impact – its unknown creator will split the formula into nine pieces, hide them in the past, and commit suicide.
In the present day, Andrei Sator has been able to find all nine of the components that were sent back through time, the final one being ‘241’, which is the item recovered in the opening Opera House action scene, lost again afterwards, and rediscovered by The Protagonist in the highway chase sequence.
Sator’s doomsday plan
With the algorithm completed, Sator intends to use it to trigger time inversion and destroy humanity. This will happen when he himself dies, as the algorithm is linked to a dead-switch activated by his fitness tracker.
Sator wishes to bring an end to everything as he has terminal pancreatic cancer, and believes if he has to die, then everything should die with him.
Tenet believes that Sator’s plan will come into effect on the 14th (the month is unspecified). This is considered by his wife, Kat, to be the last time he experienced genuine happiness, as Sator disappeared for some time the following day.
Using Priya’s time inversion turnstile, members of the Tenet organisation are able to go back to the 14th. They learn that Sator’s forces will seal the completed algorithm in an underground vault at the ‘Hypocenter’ (a space used for subsurface nuclear detonations) using an explosion.
At the Hypocenter, Tenet splits into two teams; the red team moves forward through time, while the blue team is inverted. They attack in a ‘temporal pincer movement’, creating a large distraction battle in order for a splinter cell of Ives and The Protagonist to infiltrate the Hypocenter and take the algorithm.
Simultaneously, Kat travels back to Vietnam to be with Sator on his final day of happiness, aiming to ensure he does not know of Tenet’s attack on his forces, and deal with him once his dead-switch can no longer trigger the algorithm.
The Protagonist and Ives infiltrate the Hypocenter amidst the chaos. Inside they find a locked door. Behind it is one of Sator’s soldiers preparing to bury the algorithm, and a Tenet corpse that has a red string attached to his bag. Sator’s soldier attempts to kill The Protagonist, but the corpse is resurrected by time inversion and is shot instead. The ‘corpse’, now alive, is able to unlock the door for The Protagonist and Ives to grab the algorithm.
Kat, unable to let Sator remain oblivious, reveals that Tenet is in the process of foiling his doomsday plan. She shoots him in the chest with a concealed handgun, although Sator dies only after being pushed off the side of his yacht (a tumble aided by a little slippy sunscreen). Kat then dives off the side of the boat, which means she is actually the woman she previously believed Sator was cheating on her with. This sequence also indicates that Sator never disappeared, but was actually killed by Kat herself.
Tenet secures the algorithm before the explosion at the Hypocenter and – crucially – before Kat takes Sator’s life into her own hands. The algorithm device is split into three pieces, and taken by The Protagonist, Ives, and Neil. The three intend to hide the components from future threats, and it is implied they should kill themselves to prevent the knowledge spreading.
At the end of the mission, Neil reveals that he was actually recruited by The Protagonist in the future, and that – from his perspective – they have known each other for several years. Neil’s timeline runs backwards compared to The Protagonists. This explains how Neil was able to know The Protagonist’s preference for Diet Coke when they first met.
In this same scene you can see a red string attached to Neil’s bag. This indicates that he is the soldier who takes the bullet meant for The Protagonist in the Hypocenter, as well as the soldier who saves him from the inverted bullet during the Kiev Opera House siege. Neil explains that he plans to revisit the Hypocenter using time inversion to make sure he can open the locked door, as he’s the team’s best lockpicker, which thus seals his fate. Essentially, The Protagonist’s first mission with Neil is Neil’s final mission with The Protagonist.
Neil also hands his piece of the algorithm to The Protagonist before heading back in time, suggesting that The Protagonist will know the location of two-thirds of the device.
The Protagonist’s fate
In the final scene, The Protagonist reveals to Priya that he himself will set up Tenet in the future, and so in effect recruited both himself and Priya. He then shoots and kills her, as she remains a threat to the life of Kat. People with knowledge of time inversion are seen as a threat by Tenet, and so technically should be eliminated, but The Protagonist has developed an emotional connection with Kat and thus breaks the rules that he will eventually dictate.And that is Tenet’s ending explained. For more, check out what we learned about Tenet from its cast and director, as well as our full Tenet review.