Watching 'Tenet' for the second time. And it's not about time travel. It's about reversal entropy. There's a 'professor that tries to explain it on youtube but from what I can see of the first few minutes of his video, he's tried many times (about 3 days involved in recording at least). He actually makes a 'pig's ear of it' so there's no reason to give you the link.
'Tenet' is a spectacular movie based on the idea of reversing 'entropy'. So, 'Tenet' is a film dealing with reversal entropy where things which belong to the 'other reverse universe' can go backwards under the guidance of thought. On the other side of those turnstile doors is a mirror universe where time runs backwards (Time Inversion) and that's where the reversed entropy objects can be found. In that case "Bravo!" It's a wonderfully cinematic film about that - though the characters are shallow and the dialogue is worse. You must suspend your disbelief to enjoy this movie...(turn off your brain).
And how about time-travel? The Grandfather paradox? Different versions of yourself? Look at this video... (it gives you many thoughts and ideas about the film).
TENET EXPLAINED: Time Inversion, Entropy and more! | Tenet (2020) | Film Analysis | TBFR
And there's a nice one about the storyline, too.
BUT for me, all these are nice ideas but they don't hold up well, there are too many loopholes... for example, Time Inversion... you don't poop in the toilet, you collect your poop from the toilet... yuck! And what about the bit in the movie where the gun pulls the bullets out of the wall? Well, you push the trigger to allow the bullet to leave the gun, so, in order to 'catch' the bullet, you need to do the reverse action with the trigger, which means the bullet goes into the gun when you 'release' the trigger? Plausible. (The video around 10-12:00 deals with the problems of the bullets and MAN there are some problems!) But I'm telling ya, don't stay in the 'inverted universe' for long because you'll have to eventually go for a pee or poop...
So, do you want a plausible story? Humanity H2O! It's so plausible... actually it's been happening for centuries and you don't even know it! (Hypothetically, of course).