Frances Beckett

 was one of the nuns at St Matilda's College in 1977. She didn't like Emily Shaw becoming the dean of the College. She then got annoyed at the Fourth Doctor's theories. Sister Frances trapped Leela, the Doctor and Emily in the crypt of the chapel. When the Doctor removed Lynn Pickering from the spaceship, she forced Megan Matthews to become a pilot. She then sacrificed her life so that the spaceship could take off without destroying the world. (AUDIO: The Cloisters of Terror)