The Inferno is the nickname given to a project to penetrate the Earth's crust to penetrate pockets of Stahlman's Gas at its core, which is theorized to be able to provide endless energy. Professor Stahlman himself is ill-tempered and obsessive about any interference with the progress of his work. Sir Keith Gold, the project director, is concerned about this and tells Petra Williams, Stahlman's assistant that he is calling Greg Sutton, an oil drilling expert, to consult on safety issues. UNIT is overseeing security at the project, and the Third Doctor is here too. He is using the output of the project's nuclear reactor to power experiments on the TARDIS console, which has been made portable, hoping to end the exile on Earth imposed on him by the Time Lords (The War Games).
The project, however, has its own problems. Slocum, a worker repairing one of the drill pipes encounters a toxic green slime seeping out of the pipe that rapidly mutates him into a feral creature, which goes berserk and kills one of the other workers. While the Brigadier and Sergeant Benton investigate, the Slocum creature enters the reactor control room and attacks Bromley, the technician there, then pushes the reactor power to maximum power.
At the same time, the Doctor is using the TARDIS console, and the energy rush shifts the console and the Doctor into a dimensional void, from which he is barely saved when Liz manages to cut the power. Alarms go off as the drill is in danger of shutting down due to the overload. When the Doctor reaches the reactor control and starts to lower the power output, Slocum bursts through a door, into the room, ready to kill....
The Doctor tries to calm Slocum down while he screeches horribly, but a UNIT soldier, Wyatt, tries to shoot Slocum and is attacked. Slocum eventually collapses, as does Wyatt, and the former's body is so hot that it scorched the wall where he fell. The Doctor voices his concern that Slocum's mutated form seems to be filled with some strange power. He has also heard the screeching before — during the 1883 explosion of Krakatoa. Benton informs the Brigadier that Bromley and Wyatt have gone missing, before their medical checkups, and the Doctor encounters Wyatt, who is in the process of mutating, on the cooling towers. Wyatt falls from it, to his death, but the Doctor does not notice the mutated Bromley on the towers as well.
The drill has been bringing up more quantities of the green slime, but Stahlman dismisses any connection between that and the incidents at the project. When a jar of the slime starts to bubble over, he quickly grips it, superficially burning his hand. Stahlman places the jar in a box which he orders frozen, and secretly sabotages the project computer, by stamping on the computer's micro-circuit, which was predicting disaster. He also orders that the power be cut off to the Doctor's hut, so that the drilling can be accelerated, with penetration occurring in 49 hours. The Doctor, in the meantime, sends Liz away on a wild goose chase while he hooks up the console again. Noticing the power drain, Stahlman cuts the power to the console just as Liz and the Brigadier rush back to the hut. As they look on, the Doctor, his console and his car, Bessie, vanish into thin air...
The Doctor arrives in a parallel universe. On this world, a Republic of Great Britain exists, and is run by a fascist regime after the execution of the Royal Family, possibly in 1943. The Inferno project is also ongoing, though having progressed farther in this reality. The security is provided by the 'Republican Security Forces' (RSF), a parallel universe version of UNIT. Personnel include Brigade Leader Lethbridge-Stewart, a sinister man with an eyepatch, Section Leader Elizabeth Shaw, and Platoon Under Leader Benton.
The Doctor, captured and under interrogation, tries to convince the parallel versions of his friends that he is from another universe, but they believe he is trying to feign insanity. Here, Director Stahlman has also been infected with the green slime, and is mutating. The Doctor is placed in a cell with a sedated Bromley, but the alarm is raised when Bromley awakens fully mutated. Escaping, the Doctor makes his way to the main control room to stop the drilling but is discovered. The Doctor pleads for them to stop, telling them that the screeching is the sound of the planet "screaming out its rage", with Stahlman holding him at gunpoint, with the countdown just less than 10 seconds before penetration...
As Stahlman holds a gun on the Doctor, penetration is achieved, an explosion is heard and an earth tremor rocks the installation. The temperature rises rapidly as more green slime oozes out of the cracked pipes. As the Doctor and the parallel Sutton try to contain the explosion, Stahlman, now fully mutated, attacks them. They manage to escape, leaving Stahlman behind a heat shield with the bodies of the unconscious workers. Stahlman rubs the slime on their faces, mutating them as well. There are seismic disturbances all over the country, and the Doctor explains that now that the crust has been penetrated, the planet will soon revert back to the gases it sprung from, saying they have "reached the point of no return,", and that they will never be able to plug the hole, as nothing can withstand the immense pressures of the mantle.
The Doctor tries to convince the others that he can stop this from happening in his own universe if they will help him to return, and shows them the TARDIS console. The Brigade Leader demands that the Doctor save them, too, but the Doctor says that they do not belong in the other universe. Refusing to accept this, the Brigade Leader orders everyone back to the control room, where Stahlman and his fellow mutants attack, infecting Benton as well…
Finally agreeing to help the Doctor, the group fights their way out using fire extinguishers to paralyse the mutants. By this time, the sky has turned red and the heat is overwhelming. The parallel Petra, with Sutton's help, manages to feed power to the TARDIS console. At the last moment, the Brigade Leader snaps and threatens to shoot the Doctor if he doesn't save them, but is gunned down by Section Leader Shaw. As a wall of lava sweeps towards the hut and the others watch the end coming towards them, the Doctor transfers back to his own universe…
The Doctor is back but unconscious in a healing coma, and there are only three hours left before penetration zero. When he awakes, the Doctor goes to the main control room and tries to smash the controls. He is unsuccessful and has to be restrained, but manages to tell Liz to put a new circuit into the computer that Stahlman had sabotaged. Liz does so, and the computer advises drilling be stopped at once. In the meantime, Stahlman orders everyone out of the drill head area, then when they are gone he picks up a handful of slime and rubs it into his face, causing himself to completely mutate. The Doctor escapes from the sickbay and returns to the control room, dealing with the Bromley mutant on the way. As Sir Keith struggles with the decision to order a shutdown, the Stahlman mutant emerges and has to be subdued with fire extinguishers. With seconds to go, Sir Keith orders that the drill be shut down and the shaft filled in.
Later, Sir Keith informs the Doctor that the project is being abandoned and everyone is leaving. The Doctor announces that he, too, is leaving. The Brigadier and Liz protest, and the Doctor sharply tells the Brigadier that he reminds him of his fascist counterpart. The Doctor activates the console and vanishes; on this occasion, Bessie is left behind. A few minutes later, the Doctor appears at the door of the hut with mud on his clothes and a banana skin on his shoulder, having only made it as far as a nearby rubbish dump. Suitably chastened, he asks the Brigadier to help him retrieve the console, which has landed in a somewhat inaccessible position, much to Liz's amusement.
Doctor Jon Pertwee (Third Doctor)
Companion Caroline John (Liz Shaw)
Nicholas Courtney — Brigadier Lethbridge-Stewart/Brigade Leader
John Levene — Sergeant Benton/Platoon Under Leader Benton
Olaf Pooley — Professor/Director Stahlman
Christopher Benjamin — Sir Keith Gold
Sheila Dunn — Petra Williams
Derek Newark — Greg Sutton
Ian Fairbairn — Bromley
Walter Randall — Harry Slocum
Derek Ware — Private Wyatt
David Simeon — Private Latimer
Roy Scammell — RAF Sentry
Keith James — Patterson
Dave Carter, Pat Gorman, Walter Henry, Philip Ryan, Peter Thompson — Primords
Writer Don Houghton
Director Douglas Camfield
Barry Letts (episodes 3-7, uncredited)
Script editor Terrance Dicks
Producer Barry Letts
Executive producer(s) None
Production code DDD
Series Season 7
Length 7 episodes, 25 minutes each
Originally broadcast May 9–June 20, 1970