Well, the Doctor might be suffocating right now. That would suck.
Or he got the environmental controls reversed at the exact right time. Or the Minyans did something. Either works. Point is, the gas got sucked up the vents and into the guard station. Those guys are dicks and had it coming what with their skyfalls and human sacrifices.
That would be where Idas’s father is. He doesn’t see what can be done to save the older man because there just isn’t time. But the Doctor can because he’s a Time Lord and doesn’t believe in magic since that’s just something people can trick you with to keep you in line. So, he, Leela, K9, and Idas go off to rescue Old Man Idmon. K9 quickly maps out the caves, and there’s a clear route to the citiadel, a structure identical to the Minyans’s ship. The Oracle works there with the guards and the Seers, whoever they are.
Since the episode’s runtime probably needs padding, the Doctor sends K9 to find the Minyans, so we get some shots of K9 rolling down various caverns.
So, why not take the direct route? Idas says it’s guarded by invisible dragons and even the guards can’t go that way. The Doctor has his own powers, meaning he knows science, so first he has Leela shoot out the forcefields and then get to the antigrav chamber because, according to the Doctor, there’s no gravity at the center of this planet.
I don’t think that’s how gravity works, but it does allow for a cheap special effects shot of Leela, Idas, and the Doctor “floating” downwards.
So, they rescue Idmon and the other slaves, right? Nah! They got captured right away and taken to see the really slow human sacrifice. If it were quicker, then Idas couldn’t push his dad out of the way of the literal hanging sword. And since that happened, Leela can grab a new ray gun and try to rescue the remaining slaves along with Idas and Idmon.
And just when things look bleak, K9 shows up with the Minyans, a much better armed group.
Sadly, one Minyan is captured. And the interrogation doesn’t go well because the Seers refuse to believe the captive is an actual Minyan because they are the last descendants of the Minyans. Oh, and the Seers are robots.
But meanwhile, the Doctor likewise realizes that the Trogs are the descendants of the previous Minyan craft. Also, Leela learns they eat rocks. But that means there has to be a processing center, and the Doctor and Leela do get pushed into a shaft by Idmon, allowing the two to get back into the citadel.
Hopefully they get out before they get pulverized.