Jennie was the sister of Tom, and a passenger aboard the spaceship Starmaker.
Unlike Tom and Captain Flint Fleet, she was fortunate enough not to witness the destruction of Starmaker by the Daleks — in which everyone on board, including the children's parents, perished. Fleet told Tom they mustn't tell Jennie what had happened.
Tom, Jennie and Fleet were the only survivors, and were forced to steal a Dalek ship to return to Earth to warn them of the Daleks' impending invasion.