Matron Casp was the leader of the Sisters of Plenitude. She controlled the experiments on the new humans in the Intensive Care Unit beneath New New York Hospital in the year 5,000,000,023. Casp used the experiments to cure other patients in New New York Hospital, where she was Matron. The hospital had great success in curing patients, but the existence of the new humans was kept a secret. Casp tried to divert the Tenth Doctor from the unit, but Cassandra O'Brien, inhabiting the body of Rose Tyler, discovered the unit and led the Doctor to it. After rendering the Doctor unconscious with a noxious "perfume", Cassandra demanded money from the Sisterhood in exchange for not informing the NNYPD about their criminal activities. Casp refused and responded to Cassandra's threat by unsheathing her claws with a menacing hiss. In response, Cassandra let free the Intensive care patients. This resulted in the quick death of Casp's colleague Sister Jatt.
Casp put the hospital under quarantine and then disappeared. She attempted to flee from the infectious patients by climbing up a lift shaft ladder in pursuit of the Doctor and Cassandra/Rose. She grasped Rose's ankle and berated them for ruining the Sister's work, to which Cassandra responded by telling her to "go and play with a ball of string". However, when a patient grabbed Casp's ankle, spreading its numerous viruses to her immunity-free body, Casp fell from the ladder and plummeted to her death at the bottom of the shaft, screaming all the way down. (TV: New Earth)