"Don't try to understand it," Clémence Poésy's character tells John David Washington's nameless protagonist early on in Tenet. It's advice worth following, because Christopher Nolan clearly doesn't give a damn if you're able to follow his film or not.
The core plot is actually pretty straightforward: a highly skilled CIA agent is recruited by a shadowy organization whose mission is to prevent World War III. With only a hand gesture and the word "tenet" to go on, he enlists the help of another agent (Robert Pattinson) and an art dealer (Elizabeth Debicki) in an attempt to get close to the man with the answers he seeks (Kenneth Branagh).
Also, people from the future have figured out how to send back "inverted" objects that can be utilized in the present.
This is explained to our Protagonist (who barely bats an eyelid) with the use of a time-travelling bullet, but that's pretty much where the exposition ends. This is clearly deliberate, and it's bloody infuriating. It's one thing to be required to pay close attention to a movie, but you shouldn't need a road-map and compass to navigate it. Nolan is a sharp, clever writer, so on closer inspection (and another couple of watches) Tenet will surely make more sense, but the obtuse nature of the unfolding narrative quickly becomes the film's biggest detriment.
Inception was complex and maybe even a tad confusing at times, but it rewarded the viewer's focus with satisfying reveals. That movie also had the benefit of strong, fleshed out characters and an emotionally engaging story, whereas Tenet's bland, mumbling leads only serve to irritate.
Washington and Pattinson are highly capable actors, but you'd never know it from their performances here. Maybe it's the unintentionally hilarious dialogue they're forced to spout (this is a movie in which characters refer to themselves as protagonists/antagonists with a straight face), but little things like screen presence and charisma are sorely lacking. Debicki fares much better, even with a mostly thankless damsel role, and Branagh gets away with some serious scenery chewing as the Russian baddie.
The action sequences are, at least, top notch. Nolan has become adept at putting together an exciting car chase/scrap over the years, and his prowess is on full display in Tenet. The time-travel conceit is used to impressive effect for some truly awe-inspiring set pieces, and even though you won't always be able to tell who is doing what and why (the finale is... quite something), there's no shortage of spectacle.
Tenet has its moments, but it's easily Nolan's worst film. Overlong, self-important and mostly incomprehensible; this muddled sci-fi thriller is a long way from being the "savior of cinema." Honestly, it'd be difficult to recommend even if we weren't in the middle of a pandemic.