(It's Volcano Day!)
Ah, the good old days of 7 part serials! You definitely had a different pace and a different sort of plot and character development. The downside, of course, is that it comes in 7 different parts, about 3 and a half hours of watching Doctor Who. I'd seen one or two parts of this before, but never the whole thing.
I have to say this is one of the better Doctor Who stories out of the whole run (although it does have what I consider a flaw, see below about hairy beastmen). It doesn't rely on special effects (not that there's anything wrong with special effects) but rather interaction and drama between characters. It does play fast and loose with the science, but that's nothing new for Doctor Who.
Clearly this was inspired by Project Mohole, a mid-1960's effort to drill into and through the Earth's crust. Inferno has an arrogant scientist type with a loayl female assistant, which is an interesting parallel to the Doctor and his assistant Liz Shaw. Both are irritable. Sir Keith the project manager bring in an outside drilling expert who is immediately snubbed by the scientist Stahlman, and the Brigadier has to keep a lid on the whole people problem. The Doctor is mainly interested in getting his TARDIS console to work (for some reason it's outside the actual TARDIS this episode) and Liz is worried the Doctor is going to get killed doing his experiments. Meanwhile, Sir Keith is off to some government ministry to try to get the project halted and Stahlman sabotages the project computer because he doesn't like what it's saying, mainly "stop drilling".
Oh, yeah, the computer - this is definitely the days before the PC. The thing is huge. Well, back in 1970 that's what computers were, huge, and this one is actually improbably small for the time. They also hadn't heard of video screens, the computer outputs onto narrow paper tape. So help me, I think the "microcircuit" removed and crushed by Stahlman was supposed to be some sort of fancy vacuum tube thing.
Anyhow, the story develops pretty nicely for awhile before the Doctor uses his TARDIS console to accidentally slip into a parallel universe which is also conducting its own inferno project, but it's much further along. They break through to the mantle, disaster ensues, and the Doctor escapes back to the original dimension where there is still (barely) enough time to stop the drilling. There are a few cut scenes during the alternative universe sequences where we get glimpses back to the original time line.
The fascist version of the UK was interesting, as were the differences in the people in the parallel world.
This isn't so much intellect saves the day as forewarned is forearmed. The Doctor doesn't foresee the utter disaster that is to come, he experiences it then has a chance to go back and save a world. Any intellect involved comes mainly from his working on the TARDIS which enables him to travel "sideways" rather than into the past or future.
I did mention there was what I considered a flaw in this whole thing, the Green Slime and the Beast Men. A green ooze comes out of the drill hole, rather than geysering out under great pressure?

