Mrs Prentice was a cleaner at UNIT HQ.
After hitting her head against an ionic vibrator in a laboratory at UNIT, her mind was connected to Zex. He was convinced by the Master to help conquer Earth. Zex possessed Mrs Prentice and used her to take over UNIT.
The Third Doctor and Mirko built an astro modulator and used it to banish Zex back to the fourth paraleptic level of the galaxy. Mrs Prentice retained no memory of what had happened whilst she was possessed, and the Doctor told her she banged her head, but will be fine now. (PROSE: Listen - The Stars)