Her people were involved in a devastating war. A human colony had established a base on its moon with a doomsday weapon to scour the colony of all life just in case the Earth Reptiles won the conflict. The Silurians dispatched Quarra to infiltrate the moonbase. She eliminated the entire crew from the data she had received. However, she was unaware of an entertainer, Jen Yates, who was not on the crew list. After her comrades were killed, Yates set off the doomsday device just before Quarra killed her.
The colony was wiped out. The sheer nature of the horror Quarra witnessed andthe guilt she felt made her assume Jen Yates' personality. She believed she was the only human occupant of the facility and was guarding it as the war against the Earth Reptiles raged. She also guarded a frozen embryo.
She eventually met the Doctor and his companions Ace and Benny Summerfield. At first happy to meet other people, "Jen" became suspicious of them when they avoided talking about certain subjects. She decided they were enemies. In the resultant fight, Ace held her back. The Doctor threatened to destroy one of the frozen embryo's she was guarding. In desperation, she unwittingly used her third eye against the Doctor.
It was then she remembered that she fact Quarra, the Earth Reptile sent to infiltrate the colony and the result of her failure. Unable to accept her actions, she walked out of the moonbase without a spacesuit, killing herself. (PROSE: Cold War)