The Doctor in Bessie

UNIT have been tasked to provide security cover at an experimental drilling project designed to penetrate the Earth's crust and release a previously-untapped source of energy, named Stahlman's Gas after its discoverer, Professor Stahlman. While there The Brigadier and UNIT find themselves carrying out an investigate after one of the maintenance technicians starts to act strangely and then kills another member of the drilling team.

The Doctor finds himself an unwanted observer by Professor Stahlman who dismisses the concerns of the project's Executive Director Sir Keith Gold and exceeds all safety margins in order to expedite the drilling. Professor Stahlman is more concerned in trying to ensure that nothing delays the moment when the drill penetrates the Earth's crust thus releasing pockets of the newly discovered Stahlman's Gas. So concerned is Sir Keith Gold he has called in Greg Sutton - a drilling expert from Kuwait - to seek his advice. But this added intrusion makes Professor Stahlman even more frustrated at the continued interference to ‘his’ project.

The Doctor, though is more interested in using the project's nuclear generator to carry out some experiments with the TARDIS console in an attempt in getting his time machine working again after the Time Lords took the ability to pilot it away from him as part of his imposed exile on Earth. However, at a critical moment while carrying out a test, the power supply suddenly increases – due to a technician called Slocum, who is turning into some form of sub-human primordial monster, attacks the base's power room and increases the output level. Due to the sudden surge of energy and The Doctor and the TARDIS console vanish. They reappear moments later when Liz Shaw manages to cut the power.

Liz Shaw

The Doctor heads for the drill-head control centre to find out what caused the power surge. On his way there he spots one of the technicians on top of one of the gasometers. The Doctor discovers that the technician is turning into the same sort of primordial creature. The technician tries to attack The Doctor but instead topples from a gantry to his death.

Back at the drill-head a strange green slime is discovered seeping out of an output pipe. A sample is collected so it can be tested. However, Professor Stahlman picks up the vial just as it is about to crack, and he gets some of the contents on his hand. With the computer warning that drilling must be stopped immediately, Professor Stahlman claims its announcement is inaccurate and that the drilling rate must be increased. Those around him have no choice but to follow his commands even though Professor Stahlman is no longer quite himself – due to the infection spreading up his arm. Professor Stahlman‘s continued refusal to listen to the warnings from the computer and those around him prompts Sir Keith Gold to travel to London to get authority to close down the project.

Professor Stahlman

Meanwhile back at the workshop, where The Doctor is carrying out his experiments on the TARDIS console, The Doctor makes preparations for another test run. After being sent on an errand Liz realises that she has been tricked by The Doctor to prevent her from stopping him. Liz and The Brigadier hurry back to The Doctor’s workshop but arrive to late as The Doctor, the console and his car Bessie vanish into thin air...

The Doctor wakes up, apparently in his workshop. However, when outside he discovers that not all is well. After being chased and shot at by troops he finds himself being arrested by someone who appears to be Liz Shaw but is instead a look-a-like who calls herself Section Leader Elizabeth Shaw. The Doctor is accused of being a spy and is integrated before being brought before Brigade Leader Lethbridge-Stewart, a sinister man with an eyepatch. The Doctor realises he has slipped sideways into a parallel world to the Republic of Great Britain which is being run by a fascist regime. Here, he discovers that the same project is being carried out and that Director Stahlmann is as pig-headed as his counterpart. He also discovers that Sir Keith Gold was killed in a car crash while returning from London with orders to stop the project.

As alarms start to ring out The Doctor realises that he has arrived at a point where the Inferno project has progressed further in this reality and that there is just a few hours remaining before penetration. He appeals to everyone to stop the drilling, but is ignored and is locked up in a cell next to another containing an infected technician who awakens and tries to kill The Doctor. He manages to escape and arrives in the main control room just as penetration takes place. As earthquakes rock the centre they are forced out of the drill-head area by the heat and increased flow of green slime. But Director Stahlmann stays behind and along with his technicians they become infected by the green slime, turning them into hideous primordial creatures. A battle then follows in which The Doctor discovers that fire extinguishers make a good weapon against the creatures. He also realises that this world is doomed. He pleads to be allowed to return to his universe so that he can at least try and save it. To achieve this he demonstrates the TARDIS console for Brigade Leader Lethbridge-Stewart and Section Leader Elizabeth Shaw. The Doctor though finds himself caught in a dilemma when Brigade Leader Lethbridge-Stewart insists that The Doctor take them with him.

Professor Stahlman, Petra and Sir Keith

As Director Stahlmann and the infected technicians emerge from the drill head and as a tide of lava approaches the workshop, Section Leader Elizabeth Shaw realises her world is doomed and so helps The Doctor, to escape back to his own universe, by shooting Brigade Leader Lethbridge-Stewart when he tries to stop The Doctor.

Back in our universe The Doctor is discovered by Liz. But he seems to be in some sort or coma. The Doctor though is able to mutter to Liz to reverse all the systems. The Doctor eventually recovers and finds that Sir Keith Gold is not dead. As events in this world are different The Doctor realises it can be saved. He hurries to the drill head and tries to warn of impending disaster. At first he is disbelieved and Sir Keith Gold struggles with the decision to order a shutdown. But The Doctor’s warnings words are borne out when Professor Stahlman’s infection is revealed. As he is eventually transformed into one of the primordial creatures, The Doctor, aided by consultant Greg Sutton, kills Professor Stahlman with ice-cold blasts from fire extinguishers. With seconds to go, Sir Keith Gold orders that the countdown be aborted so stopping the drill and saving our version of the world from the same fate that befell the parallel version.