GPT4 explains it pretty clearly, I think:
"Tenet" is a 2020 science fiction action-thriller film written and directed by Christopher Nolan. The story revolves around concepts of time manipulation and inversion,
making it a complex narrative that can be challenging to understand on first viewing.
Here's a basic summary:
The protagonist (referred to as "The Protagonist", played by John David Washington) is a secret agent who gets involved in a mission to prevent World War III. He is introduced to a concept called "inversion," where the entropy of people or objects can be reversed, making them move backwards in time.
His mission leads him to cross paths with a Russian oligarch named Andrei Sator (played by Kenneth Branagh), who is collecting pieces of an algorithm that can invert the entropy of the entire world, effectively reversing time and destroying the present in favor of the future.
The Protagonist also meets Kat (played by Elizabeth Debicki), Sator's estranged wife, who becomes a crucial part of the mission. The Protagonist and his partner Neil (played by Robert Pattinson) use inversion to their advantage in several action sequences, including a car chase and a final battle at Sator's secret city, where they successfully get the algorithm and prevent the destruction of the present.
The twist at the end of the film reveals that Neil was recruited by a future version of The Protagonist, and that they have been working together for much longer than the duration of the film's events. This means that the organization "Tenet", which they work for, was created by The Protagonist himself in the future. Neil's character is seen sacrificing himself to ensure the mission's success, highlighting the theme of fate and predestination in the movie.
"Tenet" is a complex film that uses its time manipulation concept to construct a narrative that loops in on itself, with events and characters revealing their true significance only as the story progresses or even after the film ends. Nolan's movie plays with the concepts of time, fate, and free will, requiring viewers to actively engage with and decipher its narrative structure.