A. St. John D. Caldera

Professor A. St. John D. Caldera was a neurologist whose mind was stolen by Skagra at the Think Tank space station. He died in the destruction of the station, but not before cluing the Doctor in on Skagra's plans. (WC: Shada)

Behind the scenes

 * In the novelisation, Caldera becomes a woman, Daphne Caldera. Her speciality is 6-dimensional wave equations, instead of neurology.
 * A is a sunken area left behind by a volcanic eruption. The names of Caldera and the other Think Tank scientists are all derived from locations or geographical features associated with the volcanic Greek island Santorini.