Christopher Nolan’s most recent mind-bending thriller Tenet leaves a lot of clarification questions - one being why Kat isn’t wearing a mask on the yacht in the finale. Nolan is well known for his confusing time-related plots, as explored in critically acclaimed films like Inception or Interstellar. The high-concept nature of the three films has even inspired an online theory on Tenet, Inception, and Interstellar being directly connected through plot points and characters in a shared “Nolanverse.”
The primary mission in Tenet involves The Protagonist (John David Washington), Neil (Robert Pattinson), and Kat (Elizabeth Debicki), as they invert themselves in the time continuum to travel backward - though not technically using time-travel - to stop Kat’s husband Sator (Kenneth Branagh) from destroying the world with a Time Bomb. The climactic scene sees Kat disguised as her past self to kill Sator on his yacht while The Protagonist and Neil engage in a “temporal pincer movement” to secure and destroy the algorithm the villain is using to catastrophically destroy time on Earth.
The time inversion aspect of Tenet is complicated, but even more confusing is when a character is reverted to forward-moving time progression. Many wondered why Kat wasn’t wearing a mask in her final confrontation with Sator in the character’s past. Not only does an inverted person move in reverse but their oxygen is also breathed invertedly too, so they must wear an oxygen mask to not choke. The reason Kat wasn’t wearing a mask was simply that she was no longer inverted. Once they had gone back to the appropriate point in time, Neil brought her through the turnstile, which allowed them to progress forward in time again and not require a mask.
An inverted Kat also wasn’t wearing a mask in the Freeport building either, though this can be explained by the inverted characters being in a sealed room. Once outside with the natural elements, their breathing is obstructed. This is why The Protagonist and Sator must wear a mask when in the inverted car chase outdoors. The oxygen mask is a device Tenet uses to tell the audience whether a character is inverted or not without explicitly stating it. This explains why certain mask-wearing characters from early on that The Protagonist encounters (such as unknowingly fighting a future version of himself) are revealed to be inverted.
The final confrontation with Kat and Sator brings up several questions on her future, but solidifies Neil’s statement about “what’s happened, happened.” The movie comes back around when early on Kat recalls how “free” a woman who jumped off of the boat in her memory was, and it's eventually revealed the woman was actually Kat from the future after killing Sator. Now her mission is complete and she can be with her son, Kat can move forward; though it raises the question about how two different Kats will be existing in the same timeline considering future Kat can’t propel herself into the timeline she came from. Tenet shows that Priya attempts to have her assassinated for knowing too much, but The Protagonist prevents this when he realizes he was the founder of Tenet.