Gretchen Alison Carlisle was a human soldier of the Combined Galactic Resistance. With the Twelfth Doctor, Clara Oswald, Ross and Journey Blue, she was miniaturised and sent inside "Rusty", a Dalek that had allegedly turned good. Their mission was to enter through the eyestalk, find out what had caused the change and to help the injured creature.
They found a crack inside the Dalek, which the Doctor fixed. This did stop the radiation leak within it, however it also caused "Rusty" to turn back to normal and contact a nearby Dalek flying saucer. The group then planned to turn "Rusty" good again. As part of the plan, they split up, with the Doctor going to the Dalek mutant while Clara and Journey went to the memory banks. Gretchen volunteered to sacrifice her own life to enable them to reach their respective destinations. Implanting a grappling hook into the Dalek, she was then shot by the Dalek's antibodies. Unexpectedly, she immediately found herself in a tea room with a woman who introduced herself as Missy and offered Gretchen tea. Missy identified the place as "Heaven". (TV: Into the Dalek)
Before she sacrifices herself she asks the doctor to "do something amazing and name it after me". Although seemingly unconnected, the tenth doctor stated he once named a galaxy 'Alison', which is Gretchens middle name (TV: the end of time)