They reached a cavern where Norna was being held in a purple force field by what looked like a bunch of giant woodlice. “So they're Tractators,” the Doctor whispered. Tegan entered the cavern from the other side. “No, Tegan, get back!” he hissed.
She obediently backed away, and the Doctor and Lilith ducked down behind a pile of black globes. When the put his head up again, a Tractator got him in a purple field and dragged him out to join Norna.
Tegan shook her phosphor lamp vigorously, and then threw it. There was a big green flash and the Tractators scattered, releasing the Doctor and Norna from the force field. Lilith rushed out from her hiding place.
“Are you all right?” Tegan asked.
“Get her out of here. We'll hold them off,” the Doctor insisted. Tegan started to protest, but she shoved his lamp into her arms. “Out!”
He turned to Lilith. “Does that blaster of yours have a stun setting?”
She rolled her eyes. “So now that we’re in actual trouble I can use a weapon.” She took out her blaster and set it to stun.
The Doctor dug through his pockets for something useful, then pulled Lilith behind a giant globe as the Tractators returned. The creatures made the globe move with their purple force field.
“Oh, no.” The Doctor struggled to stay behind the globe while Lilith took a shot at one of the Tractators. “On the other hand, with a touch of spin…” He aimed the globe at the Tractators rolled it to hit them. The force field vanished. “How's that?”
“Very nice. This is where we run.” She grabbed his hand and pulled him out of the cavern.
The Doctor spotted a green glow coming around a corner. “Ah, the proverbial light at the end of the tunnel.”
Then he and Lilith were enveloped in purple. Lilith swore in Gallifreyan.
“Language!” the Doctor scolded.
They fell forward and started to be dragged backwards. “Oh, I don’t know. I think it’s warranted in this situation,” Lilith snipped.
Tegan came around the corner. “Hold on, I'll give you a hand.”
“No, stay back!” the Doctor warned, but Tegan grabbed his hand anyway.
“It's all right, Doctor, I've got you. What's happening?”
“Some sort of gravity beam from the Tractators,” he said, scrambling for purchase. “Go, Tegan. Get everyone back to the research room!”
The Doctor and Lilith were tugged back very quickly, pulling Tegan’s hand away.
“Got any sort of plan, Dad?” Lilith asked, trying to grab a hold of something.
“Oh, plenty. Nothing that quite fits the gravity of the situation.”
“This is really not the time for your bad puns! Can you see the Tractator that’s got us in the beam?”
The Doctor nodded. “It’s just down the tunnel.”
Lilith flipped herself around and shot at where the Tractator was. There was a yelp, and the gravity beam released them. The Doctor and Lilith stood. “Good shot.”
“Why, thank you.” They started back towards the ladder to the research room. “If only we knew more about damned things.”
“I suspect Turlough can help us with that, as soon as we get back.”
Lilith heard the sound of the Tractator's gravity beam. “Get back.” They both backed against the wall. “Can you see what it’s doing?”
“There's someone up there it wants to get down. I'm going to try an experiment.” The Doctor pulled a cricket ball from his pocket.
Lilith poked her head around the corner too see the Tractator approaching. “Too late. We want to live, we’ve got to run.” They retreated back into the tunnels. “We've got to find the way out.”
“Well, sometimes it's easier to look for the way in and then work backwards.”
“Would be easier if all these tunnels didn’t look identical.”
“Oh, but they aren't.” The Doctor picked up chippings from the floor. “What do you make of these?”
Lilith raised an eyebrow. “Are they not bits of the rock that was here before the Tractators carved the tunnels?”
“These chippings have recently been machined from these walls.”
“These termites have machines?”
“Functionally advanced ones, at that. They're creating an extensive and elaborate tunnel system. Oh, insect-like they may be, but they're no ordinary insects. They have highly refined powers of abstract reasoning. These Tractators must be very intelligent beings indeed.”
“Just what this trip needed,” Lilith grumbled. “Giant, intelligent bugs.”
The Doctor wet his finger and checked for a breeze. “You're never without a sense of direction while there's an airflow. Air flows from A to B. Usually you want to be at B. Or at A.”
“Great, and which is the TARDIS? A or B?”
“Yes, well, I think you can forget about the TARDIS. It's probably scattered in little pieces across the whole of Frontios.”
Lilith narrowed her eyes at him. “You do realize this means our only way off this planet is my vortex manipulator and Rassilon knows if I can get the damn thing to take all four of us somewhere safe.”
“Yes, well.” The Doctor kept walking. “Wait,” he said, stopping abruptly. “That's the sound.”
“What is it?”
“The machine that did this.” The noise got louder. “It seems to be coming this way. Tractators dead ahead.”
“This is ridiculous. If you ask me—”
“No one is, Lilith, so shush.”
“Rude,” Lilith sniffed.
The whirring got louder, but they also heard a beep and turned to look. “Excavating machine or Tractators. Take your pick,” the Doctor said.
Lilith sighed. “Termites it is.”
They walked into what Lilith guessed was the Tractator’s lair, a huge area with metal bracings and a spherical cage in which Plantagenet was sitting. “It's a bit of a problem this, Lilith.”
The tunneling machine came up behind them. The head of a man was wired up inside it.
“That's disgusting.” Lilith gagged. “Is that a corpse?”
Whatever it was, the machine backed them up towards the Tractators.
“Not exactly. There's a living mind enslaved in the middle of that lot.”
“That face. I recognize it from somewhere.” She frowned.
The Doctor nodded. “It's Captain Revere.”