Casp was the Leader of the Sisters of Plenitude, she controlled the experiments on the humans in the secret Intensive Care Unit beneath the Hospital on New Earth during the 500,000,001st century. Casp was using the experiments to cure other patients in the New New York hospital which Casp was Matron of. The hospital gained much success from the apparent curing of patients, but the secret of the Intensive Care unit was still kept. Sister Casp tried to divert the Doctor from the unit, but Cassandra, inhabiting the body of Rose Tyler, discovered the unit and led the Doctor to it. Cassandra then accidentally let free the Intensive care patients, and 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. However a patient grabbed Casp's ankle, spreading its numerous viruses to her immunity-free body and Casp fell from the ladder and plummeted to the bottom of the shaft, killing her.