Christopher Nolan Tenet (2020) features one of the most complex time mechanics in cinema. Understanding the ending requires grasping the film unique rules about time.
Tenet Time Mechanics
Tenet introduces "time inversion": inverted objects can only interact with normal objects in specific ways, inverted people age backward from their perspective, and an object can be both normal and inverted at once (entanglement). The film uses this concept without paradox - events are fixed, not changeable.
The Algorithm
The Algorithm is a device that if activated would invert time for the entire world, ending all life. Its inventor created it to prevent someone else from using it to destroy the future.
The Opera Siege (Opening Scene)
The opening opera siege is actually the climax of the film timeline. Protagonist is there to retrieve a piece of the Algorithm but an inverted version of himself helps retrieve it, the Protagonist gets the piece then his past self gets it back, and this creates a temporal loop.
The Final Mission
The climactic scene involves both normal and inverted forces fighting simultaneously. Protagonist and Ives use a "pincer" movement - one going forward, one backward - to retrieve the final Algorithm piece from the hypocenter.
The Ending
Kat shoots Sator freeing herself from his control. Algorithm is reunited but not activated. Future consequences mean Kat and Protagonist are now "free" from the timeline problems. The paradox is avoided because the Algorithm is hidden, not destroyed, preventing any future from using it.
"Tenet"
The word itself is a palindrome - it reads the same forward and backward. This represents the film core concept: time flows both directions simultaneously, and understanding this is the key to the entire story.