Now before I start I'd like to point out, 5.7 million people tuned in to see this truly spellbinding episode, almost 2 million more people years later would tune in to watch The Twin Dilemma!! Ratings are really no indication of the quality of an episode.
The parallel Earth setup allowed the production team to have a field day with Part 6.
The Primords are breaking through, and there is an agreement to help the Doctor return. It's a desperate fight against the Primords and the heat. Petra and the Brigade Leader try to connect the power up for the Doctor to power up the TARDIS. Back on Earth progress is being forced through by Stahlman and the project there pushes ahead. Petra manages to connect the power, but it's erratic, and time is running out for the Doctor as lava flows all around.
A totally stunning episode, it really is so good. Arguably the standout Pertwee episode, it's that good. The finale is unbelievable, the Brigade Leader and Liz are superb, they play out a wonderfully dramatic scene, no over acting. The scenes of terror and screams work so well, and the final scenes of molten lava are brilliant, so much doom...
The external scenes are fantastically shot, watching it, you'd swear the air was stifling and not breathable.
I can't think of a better individual Pertwee episode, Superb 10/10