Separated from her husband Alistair, Fiona Stewart now lives in her late uncle Reggie’s house. She keeps the state of her marriage a secret from her young daughter Kate, attributing Alistair’s absence to his top-secret work for the military - although Kate has managed to learn about her father’s encounters with aliens by herself. When an alien spy named Crinix B-Theta arrives at the house posing as Kate’s teacher, hoping to glean information on UNIT and the Doctor, Fiona is immediately suspicious.