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.
As part of the plan, the group 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, offered Gretchen tea, and identified the place as "Heaven". (TV: Into the Dalek) This environment ultimately perished, however, after the 3W Institute was exposed and after Missy's Cyberman army was destroyed by Danny Pink. (TV: Death in Heaven)