Yeah like Araysar said there IS an interesting sci fi concept here, what if we (present times) fuck the future up so badly by environmental destruction that they cannot "undo" it, so their only option is to reverse time so they can have a 20 million year planetary lifespan in a rich, comforting, life sustaining environment with time going backwards. Thats a really interesting idea and there are half a dozen cool sci fi movies that come from that basic construct, sort of a reverse Terminator. I mean, just think about galactic implications - what if future humanity reverses time, and then some Space Alien race comes by and decides to genocide us for fucking with the universe in such a brutal way?
Thats not Tenet. Tenet is almost the least interesting execution of the idea, in 2019 we need a McGuffin from the future to be buried for the future. Oh, and for some entirely bizarre reason its split up and embedded in nuclear weapons from 9 nations, you know, the 9 nations that have developed nukes. Oh Jesus you can just picture Chris Nolan masturbating while writing that down.
As a software engineer it really fucking annoyed me that THE ALGORITHM was represented as a series of gears all linked together. Is that what people think code looks like? I don't even understand how THE ALGORITHM would work, but its Nolan scifi so whatever its in the same boat as "this machine lets you share dreams", "this black hole sends you behind a bookshelf".