When it was released earlier in the year, I read that many people didn’t really understand it.
As such, they thought it was a terrible film.
Having now seen it myself, I actually really enjoyed it!
It’s basically a spy thriller that uses temporal physics as a plot device.
In the far future, as the earth is dying from environmental collapse, a scientist discovers a way to travel backwards through time, but later kills herself after realising the consequences.
But not after sending the 9 parts of this machine back through time for ‘safe keeping’ in nuclear bunkers which are deemed sensible hiding places due to the radioactivity.
A mysterious organisation who wants to end the world, sends instructions back in time to the younger self of Kenneth Branagh’s Russian Oligarch of where to find those pieces to bring about the end of the world by reversing the flow of entropy (essentially turning all of time backwards and so destroying ‘our’ forward flow of time).
This he agrees to – “If I can’t have her (meaning the world, and his wife) then no-one can” – as he is slowly dying from pancreatic cancer due to radiation exposure as a younger man growing up in a radioactive town in Russia.
In the near future, The ‘protagonist’, after seeing what would happen, (probably) creates the organisation ‘Tenet’ (or is at least recruited by it) and travels back in time to recruit ‘Neil’ (Robert Pattinson) in the battle against the destruction of the world.
This is achieved by setting up a “temporal pincer movement” (by those travelling both forwards and backwards through time) around the point of death of the Russian Oligarch.
The rest of the film is explaining about how that happens, from the point of view of the protagonist who slowly learns his importance in the thread that is weaved.
If you want to understand the physics of time travel better, I can recommend this
Time: A Traveler’s Guide- https://www.amazon.co.uk/gp/product/0195130960/