England, the near future. UNIT is providing security and scientific support for the Stahlman project, an attempt to tap the Earth's core for energy. But things are already going wrong…
Quest roles:
Nicholas Courtney(Brig. Lethbridge-Stewart), Derek Newark(Greg Sutton), John Levene(RSM Benton), Sheila Dunn(Telephone Operator), Walter Randall(Guard Captain), Ian Fairbairn(Doctor Chester), Derek Ware(Pigbin Josh), Olaf Pooley(Professor Stahlman), Christopher Benjamin(Jago), David Simeon(Alastair Fergus)