Inferno

"If you break through the Earth's crust now you'll release forces you never dreamed existed!"

20th Century Earth. An unhinged scientist, Professor Stahlman, is attempting the first penetration of the Earth's crust in a top secret drilling project called Inferno. His purpose? To tap into a new energy source at the core. But at what cost?

When the Doctor and his companion Liz Shaw are called in to oversee the project, he soon develops grave misgivings. Things begin to go very wrong when a mysterious green substance leaks from the drill head. A substance which turns all who come into contact with it into alien primeval creatures called Primords.

Meanwhile, the Doctor finds himself transported into a parallel universe identical to 20th Century Earth. The mystery deepens as he finds that although the place, time and people are all the same, no one seems to know who he is.

Only one thing is certain: the drilling must be stopped before the full force of the energy from the core is unleashed, destroying not only this Earth, but the one the Doctor has left behind . . .

• This is the final story in which the original TARDIS console was used by the production team.  After 7 years of use, it was beginning to show its age.  It was replaced by a newer, more reliable version.
   

• Fans will also notice that throughout Pertwee's first season that his companion, Liz, has an ever changing hair design (and sometimes color too!). 
   

• Actress Caroline John (Liz Shaw) left at the end of this season.  The decision occurred after production had finished on the seventh season, so as a result, there is no farewell scene for her character.  Her departure is briefly mentioned in the next season's opener "Terror of the Autons."
   

• A special mention should go to Nicholas Courtney, who does a wonderfully evil "Brigade Leader" (eyepatch, scar, and all).    :)
   

• Three actors appearing in this story have also been involved in Doctor Who at some other time.  Derek Newark appeared in the first ever Doctor Who story "An Unearthly Child."  In it he played the caveman-who-would-be-king, Za.  Actor Walter Randall makes a brief appearance here as Slocum, but can also be seen alongside William Hartnell in "The Aztecs" as Tonila, in "The Crusade" as El Akir and in "The Daleks' Masterplan" as Hyksos.  You can see more of him than you ever imagined in an interview session on "The Aztecs" DVD.  The third actor is Christopher Benjamin (here playing mild mannered Sir Gold).  Benjamin would later return to Doctor Who playing the larger than life character of Henry Gordon Jago in the Tom Baker classic, "The Talons of Weng-Chinang."
   

• Fans of DVD should take heart.  In a recent Doctor Who Monthly poll of fans, "Inferno" came in the top ten (and was indeed the highest rated Pertwee story in the poll).  It could well get a DVD release sometime in the near future.