UNIT is providing security cover at an experimental drilling project at Eastchester, designed to penetrate the Earth's crust and release a previously untapped source of energy. Soon, however, the drill head starts to leak an oily green liquid that transforms those who touch it into vicious primeval creatures with a craving for heat. The Doctor is accidentally transported "sideways in time" by the partially repaired TARDIS control console into a parallel universe where the drilling project is at a more advanced stage. Thwarted by his friends' ruthless alter egos, he works to save both universes.
| Writing | Don Houghton | Writer |
| Directing | Douglas Camfield | Director |
| Costume & Make-Up | Christine Rawlins | Costume Designer |
| Camera | Fred Hamilton | Camera Operator |
| Editing | Martyn Day | Editor |
| Costume & Make-Up | Marion Richards | Makeup Artist |
| Writing | Terrance Dicks | Script Editor |
| Sound | Brian Hodgson | Sound Effects |
| Sound | Delia Derbyshire | Music Score Producer |
| Sound | Ron Grainer | Music Arranger |
| Production | Barry Letts | Producer |