10 Small Details You Missed In Tenet

"We live in a twilight world, and there are no friends at dusk." This phrase was used by the Protagonist to identify his target during the film's opening setpiece and its use adds layers to the film's mystique.

It refers to a state of uncertainty and while on a meta level this can be applied to the film being hard to understand, it also speaks to the characters' general mistrust/disbelief of what they are seeing and who they are encountering, respectively.

It is a nice piece of worldbuilding and harkens back to how spies would use distinct phrases to distinguish between friend and foe. In addition to this, it is possible that the use of the word twilight hints at the possibility of finding allies, and the use of dusk indicates that there are none.

Following the film's release, various theories popped up in a subreddit dedicated to it. Some of the more interesting theories on the thread include:

"It’s a code for CIA, but it can also mean something if you look into it: Twilight/Dusk= hope, reborn. Dawn = despair, death"

"The tradecraft term for what they are doing when using those phrases is referred to as “bona fides”. Even more commonly used is the term “challenge and password”. As others have said, they’re used to identify people on your side"

"Maybe twilight world is referring to the state of the world the protagonist is living in. He still has to do all the stuff he has to do and believe in the outcome and not stray from his path, because if he does the world will maybe just perish and never existed."

The thread shows how open the line is interpretation, and given that Nolan likes his work to have multiple interpretations, it is possible that most of these theories are true.