[The following essay contains MAJOR SPOILERS; YOU HAVE BEEN WARNED!]
I’ll see you at the beginning, friend.
As the dust settles following the explosive climax of Tenet, a fire-forged friendship comes to an end... and, paradoxically, the very same friendship begins for the other party. Robert Pattison’s Neil knows that he must return to the past in order to save The Protagonist's life at the cost of his own—and he is happy to make that sacrifice, because he is confident that their shared experiences will inspire The Protagonist to become his mentor in the future, thus closing the loop and fulfilling their preordained destinies. This decision is perfectly consistent with the philosophy that Neil has espoused throughout the entire film: “What’s happened, happened.”
Christopher Nolan has always been obsessed with the subject of time; Memento dissects and reverses it, Inception distorts and elongates it, Interstellar shatters it on a metaphysical level. With Tenet, he distills the theme to its purest form, imagining a world in which advanced technology allows the characters to literally “invert” linearity. This does not, however, mean that their actions have no consequences; indeed, it wouldn’t be inaccurate to say that they are enslaved by fate. At several points, circumstances force The Protagonist to repeat events backwards, bringing him into conflict with his former self; these encounters are unavoidable and immutable, despite the fact that his prior knowledge should theoretically enable him to circumvent them. His tampering with causality eventually results in some rather... dramatic temporal anomalies, with far-reaching repercussions:
You thought I was working for you. I realized we’ve both been working for me.
Contrary to the existential horror suggested by that quote, Tenet’s tone is ultimately hopeful. Neil succinctly expresses the movie’s moral when he further elaborates on his personal beliefs:
What’s happened, happened. Which is an expression of faith in the mechanics of the world. It’s not an excuse to do nothing.
It’s an unusually optimistic interpretation of fatalism—one that steadfastly refuses to dismiss the notion of free will. While Tenet’s unseen villains gamble with the very fabric of reality, attempting to alter the course of history in the pursuit of selfish goals, its heroes choose to maintain order—even if their predetermined paths inevitably lead to certain doom.
What a beautifully poetic contradiction!