At the end of 2013’s Star Trek Into Darkness, it is indicated that Carol Marcus – who had earlier in the film boarded the ship under a false name with a phony transfer order – was to become a permanent member of the Enterprise crew. Fast forward three years to this summer’s Star Trek Beyond, and Dr. Marcus is nowhere to be seen.
For continuity nerds and Into Darkness apologists, this absence may have rankled, but on a recent episode of Engage: The Official Star Trek Podcast, Simon Pegg discussed why he and co-writer Doug Jung decided not to bring this character back.
It felt like we would underserve her if we included her. She might not be given the amount of screen time she deserved, so rather than bring her in and just have her be a supporting role– or the worse thing would be have her be in the film and then that character be killed. … So rather than have Carol Marcus be not used in a reasonable capacity, let’s just not include her and have her be alive in canon and ready to come back at any time.
In other words – in this movie that’s very deliberately a celebration of the original series characters – there wasn’t really a place for Carol Marcus. And, if we’re being totally honest, there wasn’t really a place for Marcus in the last story either. Her entire role in the film amounts to little more than an empty callback to Wrath of Khan (in a movie that’s already stuffed with empty Wrath of Khan callbacks) and to have a scene where she stands around in her underwear for no good reason so the studio could cut boobs into the trailer.
The more interesting point, though, is that Pegg says that even though they couldn’t see any use for her in this story, they deliberately left her out of the film so that somewhere down the line she’s still available for someone who does. This is the correct way to contribute to a long-running series. Leave options open for future storytellers to work with rather than shooting beloved characters in the head for no reason other than you thought it would be fun.
If you’re looking for an in-story explanation, though, Pegg also notes that in earlier drafts of the script there was a line about how she had left to go do preliminary work on the Genesis Project (another empty Wrath of Khan reference but whatcanyado?).