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 theorised 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.
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 Shaw 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 the man 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.