You don't need to understand it to follow the story, and given the director's reputation for detail on this sort of thing, I'm sure it holds up, but it makes "Memento" seem linear. Also, everyone has a foreign accent or mumbles so I missed a lot of dialogue that explains things.

Anyway, the protagonist, who actually refers to himself as such a couple of times, which is as close to an identifying term as I can recall, played by John David Washington, is a CIA tactical operative who is recruited into a secret conspiratorial organization or operation to stop World War 3, with time as the weapon instead of nukes. Tenet is the name of the group and its identifying password, and its membership includes Clemence Posey and Aaron Taylor-Johnson.

The film's gimmick is that in the future they've figured out how to "invert" objects, which makes them move backward in time. So instead of being dropped, they pop up into your hand. Inverted bullets pop out of a bullet hole you just noticed and reappear in your gun. Without the gimmick, it's basically a series of quests to unlock the next one, but the inverted action makes it interesting.

Washington's character recruits Neil, a British operative played by Robert Pattinson, as their trail leads to a Russian tycoon, played by Kenneth Branagh and his hostile wife, played by Elizabeth Debicki.

As the conflict escalates, Tenet and their adversaries display a device that actually lets people become inverted. To normal people, they appear to be moving backwards, and from their own perspective, they have the advantage of moving toward the past, with foreknowledge of how it's going to go, allowing them to anticipate and preempt their adversaries, leading to even more screwy action.

As a technical accomplishment, the movie is pretty amazing, and Nolan shows no sign of backsliding on his ability to maintain and ramp up tension displayed in his last film, Dunkirk. This serves to paper over the aforementioned simplistic and forced plot structure of "find the thing to get to the person, who will tell you the next thing you need to find". On the other hand, given the complexity and counter-intuitive progression of the action scenes, that might have been necessary to keep the story accessible. Also the emotional stakes don't land as well. Washington is not ordinary enough to identify with as, say, the nameless Tommies on the beach or the yachtsmen in "Dunkirk", but he also doesn't have the personal problems and interests like the protagonists of "The Prestige" or "Inception" or "Interstellar". He's a cipher who, as far as the audience is concerned, has no past or connections, certainly nothing that gives him the slightest qualm about dropping that life to disappear into the underworld of Tenet. The attempts to invest the audience in a relationship of some kind with Debecki's Cat don't really land and given the apocalyptic stakes of the mission, his concern over her personal problems is more of a distraction and irritant. The other Nolan film with similar stakes, "Interstellar", at least made love and human connections an integral part of the solution. It doesn't work like that with Kat & Washington, and she isn't super sympathetic, despite the unpleasant situation in which she is introduced. We don't care about her kid the way we care about Mal's or Coop's. Washington himself is engaging and likable even in his serious role, with combat abilities that never really make you feel like he is in danger unless time-shenanigans are going on, and you don't question why on more than one occasion, his colleagues are willing to go to great lengths or make significant sacrifices for him. Really, the main, and more interesting, relationship in the film is the one between Washington and Pattinson's Neil.

Speaking of Pattinson, I'm fairly confident that we're due for yet another round of crow-eating from the people horrified at the latest casting of Batman, and not just because he's now worked with the best Bat director. Washington brings humanity to his badass action operative role (more so, IMO, than say, Daniel Craig as James Bond), and there are times when he sounds eerily like his father.

Overall, it's a good movie, with good performances, despite the weak plot and emotional stakes. But the appeal here quickly becomes the most novel and interesting time travel mechanics in quite some time, and I can do without that other crap most of the time.