Doctor Who's first dabbling in an Alternate Universe. Professor Stahlman has discovered "Stahlman's Gas" beneath the Earth's crust and UNIT are providing security for the drilling operation to exploit this useful resource. Unfortunately, Stahlman is driven to disregard safety precautions, and things start going wrong. The drilling strikes a green goo which regresses anyone who touches it into a beastlike "primord" which is drawn to heat. The production team had been looking for an excuse to get rid of the old console prop, and this seemed the perfect opportunity... Watch it here.