Doctor Who: Inferno (1970)

Still exiled on Earth, The Third Doctor and UNIT have gotten themselves attached to a pioneering drilling project which aims to penetrate the Earth's crust and release hitherto unknown energy resources.

The Doctor is also using this as a chance to siphon off some power to see if he can take the TARDIS console - currently separated from the main body of the TARDIS - for a "test flight".

The drilling operation - nicknamed 'Inferno' - is being led by the single-minded, and highly obnoxious, Professor Stahlman (Olaf Pooley) who will truck no interference (perceived or genuine) in his scheme.

However, there are concerns on the base because of a number of unexplained deaths and the fact that Stahlman isn't listening to anyone's concerns about the safety of the process.

Unknown to all, a strange green substance has also been oozing out of the drill head and mutating those who touch it into murderous, beastial "primords" - who spread their infection by the slightest contact.

The Doctor is continuing his experiments with the TARDIS console when an accident sends him "sideways in time" to a parallel world; an oppressive, fascistic Orwellian reflection of the Earth he had come from where the drilling operation is part of a slave-labour camp controlled by an equally pig-headed Stahlman and a scarred, one-eyed version of Brigadier Lethbridge-Stewart (Nicholas Courtney).

The Doctor is arrested as a spy and even with his best efforts - because this new world is slightly ahead in its progress - he seems unable to prevent an Earth-shattering catastrophe.

Despite several faults (some unnecessary, verisimilitude-shattering CSO and taking too long to really get going), Inferno is an incredibly gripping story with the "new Earth" The Doctor travels to having strong echoes of the Mirror Universe from 1968's Star Trek episode Mirror, Mirror, with a bit of George Orwell's 1984 thrown in for good measure.

The constant noise of drilling and alarms adds a degree of claustrophobia to this very Quatermass-like story, and forces most of the characters to shout rather a lot to get their point across, which certainly heightens the tension in a strong, new direction for Doctor Who.

But for all the time it takes to tell its main story, Inferno tosses out some rather major questions that are never answered: particularly, what is the green goo and why does it change people into these so-called "primords"? Understandly, The Doctor's primary concern is averting a major disaster that could render the Earth uninhabitable - but he doesn't appear at all curious as to the nature of the super-heated substance or its effect on people!

Perhaps if more time had been spent on this aspect of the story and less on repeating information about the state of the drilling, and generally padding out its 167-minute duration, the script could have been a lot tighter.

There is an awful lot of extraneous material that a modern editor or director would snipped from the script to keep the story cracking along, and its credit to both Don Houghton's writing and Douglas Camfield's direction that the pace and tension does actually remain high throughout despite the obvious repetition and needless distractions.

We establish within minutes of the first episode that Stahlman is very driven and slightly delusional, but it isn't until the third episode - out of seven - that The Doctor gets accidentally shunted to the parallel Earth and gets to the real meat of the plot.

It is, without a doubt, this parallel universe segment at the heart of the story that makes Inferno such a memorable tale, with its alternate takes on Liz Shaw (Caroline John) as a brunette military officer, the Brigadier as a cowardly bully and Sgt Benton (John Levene) as a mindless tool of the system.

All the stuff with the Primoids, while possibly an essential element of the story, seems tacked on to raise the threat level and make it slightly more "science-fiction" and "Doctor Who".