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.
In some circumstances, the infection could be transmitted if a Primord touched a human and heat would cause the transformation to progress at a more rapid pace. Primords were resistant to gunfire. Their known weaknessess were falls from a great height and rapid application of cold, such as from a fire extinguisher. The degree of intelligence displayed by the Primords was variable; they acted primarily on instinct, but displayed signs of organisation and tactics. They made a high pitched, screeching sound, which the Doctor claimed that he had heard before during the eruption of Krakatoa in 1883.
The BBC Books sequel to the serial, The Face of the Enemy by David A. McIntee, later revealed that despite the Doctor's assumption, the parallel Earth had in fact not disintegrated, but the surface had been devastated beyond recovery. It also implied that many survivors of the disaster had since become Primords.
John Levene's portrayal of Benton as a Primord was inspired by Richard III (so nicknamed because of the Primord creature's hump).
Working titles for this story included Operation: Mole-Bore, The Mo-Hole Project (after the real-life Project Mohole) and Project: Inferno.
The opening credits for this serial were unusual in that after the initial titles, the name and part number of the serial were superimposed on footage of a lava flow, with no music.
In spite of Douglas Camfield receiving sole credit as director, Episodes 3-7 were directed by producer Barry Letts after Camfield had a minor heart attack on April 27 1970. Letts later stated that Camfield's preparations were so meticulous, that he just followed the other man's plans anyway. Camfield remained credited as director, as BBC regulations at the time forbade any person from being credited for more than one production role, and they did not want Camfield's illness to become widely known, lest it harm his career.
Derek Ware did not actually perform the scene where the mutated RSF Private Wyatt is shot and falls to his death from the top of one of the cooling towers, in case he was injured, as he was also needed for studio recording. His place was taken by Roy Scammell, who strangely also played the RSF sentry who fires the fatal shot. Ware also stated in an interview that Scammell had already signed the contract to do the fall before Ware had been cast as Wyatt. At the time it was filmed, the fall was the highest fall ever performed by a British stuntman.
Episode 6 has a small damaged section on the tape, which the Doctor Who Restoration Team replaced by painstakingly recolouring the appropriate section of the existing black-and-white film prints.
Caroline John enjoyed her role as Section Leader Elizabeth Shaw and says that it was fun playing 'baddie' Liz. She also says she hated doing the scenes when she was playing the 'goodie' version because it was boring compared to being an evil character. She was particularly upset though about the scene in which Shaw shoots Brigade Leader Lethbridge-Stewart, as she was pregnant at the time. As a result, the scene was recorded with the weapon fired from out-of-shot, after which Shaw was shown returning the gun to her holster.
During the scenes set on the parallel Earth, images (supposedly) of the UK's dictatorial leader are seen on posters. The image used is that of Visual Effects Designer Jack Kine, in homage to the 1954 BBC adaptation of Nineteen Eighty-Four where the face of Big Brother was Head of Television Design Roy Oxley (Kine had worked on the visual effects for that production).
The role of Petra was given to Sheila Dunn after Kate O'Mara was not available to play the part. O'Mara would, years later, be cast as the Rani, a renegade Time Lord. Dunn was the wife of this story's director, Douglas Camfield.
The videotapes include an additional scene in Episode 5 that was not originally transmitted in the UK, but was retained for overseas screening (and has also appeared on both the UK Gold transmissions and the BBC Video release). Set in the Brigade Leader's office where the survivors listen to a radio broadcast done by Jon Pertwee in the style of Lord Haw Haw, the scene was cut because Pertwee's voice was too identifiable. It is also interesting to note that the radio announcer names the area where the Inferno project is taking place as being Eastchester; the name is not mentioned anywhere else in the story. The scene was included as an extra on the DVD release, with the episode itself presented exactly as originally transmitted (using black-and-white telerecordings for reference when editing).