At a deep-drilling project, one of the maintenance technicians, Harry Slocum (Walter Randall), touches some green slime at the drill head and goes insane. The Brigadier and UNIT are called in to investigate after he kills someone. 

In charge of the project is Professor Eric Stahlman (Olaf Pooley) who, along with his personal assistant Petra Williams (Sheila Dunn), is trying to ensure that nothing delays the moment when the drill will penetrate the Earth's crust thus releasing pockets of a new gas - called Stahlman's Gas - which is claimed to be a powerful energy source. Sir Keith Gold (Christopher Benjamin), Executive Director of the project, does not share Stahlman's confidence. He has called in Greg Sutton (Derek Newark), a drilling expert from Kuwait, to seek his advice. 

The Doctor, who is using the project's nuclear generator to carry out some experiments with the TARDIS' console, switches on the console for a test, but, at the critical moment, Slocum, who is turning into some form of sub-human monster, attacks the base's power room and increases the output level. There is a surge of energy and the Doctor and the TARDIS console vanish. They reappear moments later when Liz manages to cut the power. 

Slocum is found and shot. However a technician, Bromley (Ian Fairbairn), and one of the UNIT soldiers, Private Wyatt (Derek Ware), seem to be suffering from extreme shock. Shortly afterwards, the Doctor sees Wyatt on top of some gasometers - he is turning into the same sort of beast-man as Slocum. He attacks the Doctor but topples from a gantry to his death. Bromley, meanwhile, has also started to change. 

Back at the drill head, Sir Keith has collected a vial of strange green slime from around number two output pipe. Stahlman picks up the vial as it is about to crack, and he gets some on his hand. The computer is warning that drilling must be stopped immediately, but Stahlman claims it is inaccurate. He also cuts off the Doctor's power. 

Stahlman removes a micro-circuit from the computer but before he can smash it, the Doctor interrupts and immobilises the Professor with Venusian Karate. However the Professor will not admit to having tampered with the computer, and when the Doctor has gone, Stahlman crushes the micro-circuit under his heel. 

Back at the outhouse, the Doctor, who has surreptitiously reconnected his power, sends Liz back to the drill head to check some epsilon co-ordinates on the computer. As soon as she has gone, the Doctor makes preparations for another run with the console. Liz realises she has been tricked and she and the Brigadier hurry back to the outhouse in time to see the Doctor, the console and Bessie vanish. 

There are 49 hours and 15 minutes to penetration and Sir Keith decides to go to London to get authority to close down the project as Stahlman, already showing signs of 'infection', will not listen to reason. 

The Doctor wakes up, apparently in the same place. Outside, he is chased and shot at by troops before being found and arrested by someone who appears to be Liz Shaw. She is, in fact, Section Leader Elizabeth Shaw (Caroline John) and she takes the Doctor, whom she suspects of being a spy, to see the Brigade Leader (Nicholas Courtney). The Doctor realises he has slipped sideways into a parallel world. Here, there are only 3 hours and 22 minutes until penetration, and project Director Stahlmann (Olaf Pooley) is being as pig-headed as his counterpart. The Doctor learns that Sir Keith has been killed in a motor accident on the way to London and that there seems to be nothing to stop Stahlmann. 

Alarms ring out as number two output pipe hits problems. In the confusion, the Doctor manages to repair the computer and it immediately advises that they should disperse the pressure and heat by creating a reverse vortex at the bottom of the drill shaft by reversing the systems. This works but Stahlmann intends to restart the drilling as soon as he can. The Doctor is taken to the security cells, from where he escapes when the man in the next cell turns out to be infected by the slime. With three minutes to penetration, the Doctor again appeals to everyone to stop the drilling, but is ignored. 

At penetration, earthquakes rock the centre and the Doctor and Sutton try to get the coolant flowing once more. Stahlmann attacks them and they are forced out of the drill head area by the heat. Stahlmann closes the heat shields and proceeds to rub the trapped technicians' faces in the green slime, turning them into sub-human monsters. The Doctor discovers that a fire extinguisher makes a good weapon against the creatures and also realises that this world is doomed. He asks that he be allowed to return to his universe to try and save it. To this end he demonstrates the TARDIS console for the Brigade Leader and the Section Leader. The Brigade Leader insists that the Doctor take them with him, but the Doctor refuses. 

Stahlmann and the infected technicians emerge from the drill head. Benton (John Levene) is attacked and turned into a monster himself. Liz, Greg and Petra agree to help the Doctor escape and manage to get the power switched on and routed to the TARDIS' console. Liz shoots the Brigade Leader when he tries to stop the Doctor, and, as a tide of lava approaches the outhouse, the Doctor finally escapes back to his own universe. 

He is found in a coma by Liz. The emergency flange blows on number two output pipe, and the Doctor manages to mutter the solution to Liz: reverse all systems. The Doctor eventually recovers and finds that Sir Keith is not dead. This world is different and can therefore be saved. He hurries to the drill head and starts smashing the equipment with a metal bar. Stahlman has him arrested but he escapes from the UNIT soldiers and returns to the drill head, where Stahlman has now transformed into a sub-human creature. He is disabled by being dowsed with fire extinguishers and the Doctor cuts off the nuclear power and initiates shut-down. With 35 seconds to go, the drill stops and the countdown is aborted. 

The Doctor makes one final trip with the console, after calling the Brigadier 'a pompous, self-opinionated idiot'. Unfortunately he travels only as far as a rubbish tip down the road.

_{Synopsis from Doctor Who: The Third Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}