Plot Summary: Still stranded on Earth by the Time Lords, the Doctor is working with U.N.I.T. at a project nicknamed 'Inferno', where a group of scientists are working towards penetrating the Earth's crust in order to tap into pockets of Stahlman's Gas, which they believe can provide boundless cheap energy,
Whilst the Doctor is not an active participant on the project, partly due to his confrontational interactions with Professor Stahlman the project director, he is using the surplus power from the project's nuclear reactor to work on the Tardis console, trying to repair it and allow himself to travel through space and time once again.
Whilst repairing a part of the drilling equipment one of the workers comes into contact with a strange green ooze that goes on to mutate him into a subhuman primordial creature who attacks several other members of the project. Unbeknownst to anyone else, Professor Stahlman is also infected.
Soon after they subdue the mutated workman the Doctor is working on the Tardis when a power surge activates the console and he vanishes in front of Liz and the Brigadier. Instead of travelling through time or space the Doctor finds himself transported to a parallel universe.
The Doctor discovers that this new Earth is a much darker counterpart of the one he knows, where Great Britain is a Republic fascist regime, the Royal Family having been executed years before. On this new Earth the Inferno project is still ongoing, though is much further along than the one on his Earth.
When he is captured by this universe's version of U.N.I.T., the British Republican Security Forces, he discovers the alternate versions of his friends. In this universe the Brigadier is the vicious one eyed 'Brigade Leader', Liz Shaw is a member of the military and is a 'Section Leader' along with 'Platoon Under Leader' Benton.
The Doctor finds out that this universe, whilst very different from his own, has had several of the same events happen, including the mutated worker. In this universe Professor Stahlman has also been infected and is beginning to act bizarrely as the effects begin to take hold. Convinced that something bad will happen he breaks out of his cell and attempts to sabotage the drilling. Unfortunately he is too late and the drill breaches the Earth's crust.
The whole facility is rocked by massive tremors and many of the scientists and staff flee. Stahlman locks himself and several scientists in the drilling room and begins to expose them to the green slime. Because of the intense heat he and all of the others mutate into Primords, bestial creatures that are more monster than man.
Discovering that the tremors are not just isolated to the facility, but are tearing the whole planet apart, the Doctor manages to convince the others to help him return to his world in order to prevent the same thing from happening there, promising to take them all with him. The group fight their way across the facility to where the Tardis is located, keeping the Primords at bay with fire extinguishers, having discovered that the cold CO2 inside them makes the creatures vulnerable.
Massive explosions hit the facility as the Doctor is readying the Tardis to travel back to his own world and the Brigade Leader pulls a gun on the Doctor, demanding that he save them all. The Doctor tries to tell him that despite his earlier promise he can't bring any of them to his dimension. The Brigade Leader goes to shoot the Doctor, but is killed by Section Leader Shaw. The Doctor activates the Tardis and disappears as a wall of lava envelops the building and the others.
Arriving back in his own dimension the Doctor falls into a coma. Awakening hours later the Doctor rushes to the drill and begins to destroy equipment in an attempt to stop them from breaching the Earth's crust. The Doctor is detained by U.N.I.T. soldier, who believe he has lost his mind.
Inside the drill Stahlman exposes himself to the green ooze and mutates fully into a Primord. The Doctor manages to kill the Primord Stahlman with a fire extinguisher and shuts down the drill before they breach the Earth's crust and cause complete global destruction.
Analysis: 'Inferno' is the final episode of Jon Pertwee's first season as the Doctor, and the first year where the show was located purely on Earth. Being limited to only telling stories in one place and time meant that the writers had to get increasingly creative when it came to coming up with new scenarios to put the Doctor in. 'Inferno' marks possibly the best attempt at this in one of the best episodes of the Classic Doctor Who era.
At the time 'Inferno' was one of the darkest episodes the series had produced, featuring a bleak story that allowed them to kill several of their main characters, albeit in the form of their parallel world counterparts.
The idea to create a story around a parallel Earth came from the series having ran out of budget at the latter half of the season, and as such the writers came up with the idea so that they could use the same sets and characters in more than one way. It might have been a decision based purely on a production and cost level, but the writers managed to craft a story that worked incredibly well, and one that never felt cheap or easy.
Subtle differences to the second Earth, such as a slight difference in fashion and the Republic propaganda posters meant that very little effort was needed to create the new Earth, yet kept the two distinct and different enough in their looks that you're never confused as to which Earth we are watching.
One of the best things about the story, though, is getting to see some of our familiar and beloved characters playing 'evil' versions of themselves. Despite being a military woman Liz is not too far removed from her other Earth self, and even saves the Doctor at the end knowing that she had no hope of living herself. The Brigadier and Benton are a completely different matter though, as the two of them are just thoroughly nasty pieces of work.
Benton becomes a Primord, so we never get a chance to see how this version of his character would play out in the final scenario, but much like the Brigade Leader he seems completely set in his devotion to the Republic and even enjoys his role.
The evil counterpart to the Brigadier is by far the best of the three of them though, with the Doctor's best friend transformed into a fascist military leader who gives orders to murder people without a second thought a stark contrast to one of the series' most beloved characters. He's visually stunning too, with the massive facial scar and eye patch means that he will certainly stick in the mind long after the episode has finished. It's a shame that he didn't somehow manage to come across to the main universe as it would have been great to have him in more than one story.
Despite a heavy presence in the story the Primords are not really the main villains or even the biggest threat, but more of an obstacle for the characters to deal with. Instead it's time that proves to be the biggest enemy here, with the countdown to the destruction of both Earths being the biggest threat. This is another factor that gives the story a unique feel, as the big monster is reduced to a minor part of the story rather than being the main focus.
The only downside of the story is that it marked the final appearance of Caroline John as Liz Shaw, who left the show before it returned for season 8 to have her child. As it is Liz never gets a goodbye scene with the Doctor, and despite only being in the show for one season she deserved more than that. As someone who is closer to be an equal to the Doctor than many of his other companions over the years her departure is definitely something that should have been given some thought and screen time.
'Inferno' has a look and a feel totally unique to itself that makes it one of the more entertaining, effective and engaging stories in the classic era of Doctor Who and a story that is still fresh and enjoyable to watch 46 years later.