Part 3

It was difficult, making their way through the laboratories in the darkness. They didn't even have any windows into the ocean to provide bioluminescence, but it was either this or get caught – which, apparently, they didn't want to do. Ian didn't really see an argument with that. The fact that they were sneaking around seemed to support this, at least.

Susan had woken them late at night, accompanied by Reina, who had claimed that the voices they'd heard weren't side-effects of the pressure after all.

Ian was very willing to believe that, but he hadn't been prepared for what Reina claimed was the real reason.

"Psychic sponges."

"What do you mean, psychic sponges?" Ian asked.

"It'll make more sense if I show you," said Reina. "Come with me."

"Hmph," said the Doctor, though his curiosity was written plainly across his face. "And why should we?"

"I need your help," said Reina. "If you help me, I'll help you get the materials to fix your scanner."

This seemed to appease the old man, and he allowed her to lead the way back into the laboratories. She had a dim torch, which she only used when absolutely necessary. Other than that, they were in the dark, half making their way by feel. Reina led them down to the laboratories, and through a series of corridors therein that Ian and the Doctor hadn't even seen during their tour earlier that day.

"Here it is," Reina hissed. "Or, some of it."

She shone her torch on a beaker, filled with some sort of sludge. It pulsed slightly as she moved the light over it.

As she did, Susan doubled over again.

"There it is again!" Susan whispered. "One of the voices."

"Hmm? Hmm!" the Doctor looked over the beaker. "I can feel it too, my child. Not as strongly as you, I think, but you're right."

"What is it?" asked Ian.

"It is as this young lady claims, I believe," said the Doctor. "Psychic sponges, indeed! Hmm! They must have evolved here over millions of years!"

"Why are they psychic?"

"So they can talk to each other!" said Susan. "Am I right, Grandfather? That must be the mumbling I can hear."

"That's what we think," said Reina. "Psychic sponges, they must have evolved sentience, and then psychic abilities to communicate with each other in the depths of the oceans."

"Why are they here?" asked Barbara.

"And why are they sad?" asked Susan.

Reina's expression was grim. "Well, you see, that's the problem. The energy we make in the plant isn't geothermal – it comes from these. We process them."

"That's horrible," gasped Susan.

"Why, that's barbaric!" said Ian.

"It's efficient," said Reina. "And so much of our technology has developed from studying them. Organic computers, efficient, sustainable energy, and psychic shields."

"And why do you need psychic, eh, psychic shields, mm?" asked the Doctor.

"So we don't hear the screaming," said Reina. "Once you're here for a day, you develop psychic shields. When the plant grew into a city, and civilians came down, we made up a lie that the voices were a short-term thing to do with the depths, and, well, people accepted it after a while."

"So why are you telling us all of this?" asked Ian. "What do you want us to do?"

"I want this to stop," said Reina. "Yandi keeps close tabs on me, he won't even let me tell Theodore about it. But I want this to stop." She turned to the Doctor. "You, Doctor, I watched you while we showed you around the lab. You are a man of science. I haven't figured out how to do it, but maybe you can help me."

"Do what, exactly?" said the Doctor.

"Let us talk to the sponges. Not just hear them, talk to them. I want everyone in the city to be able to talk to them. If people find out what is happening, and can talk to them, maybe this will all end."

The Doctor eyed the sponge in the beaker. "Give me some time with it, my girl, and I'll see what I can do."

It became evident, as Ian and the others sat there, watching the Doctor and Reina at work, that he relished the opportunity to work in a laboratory as well equipped as this. They weren't even sure whether half the instruments he was using were actually necessary for whatever he was doing, as after a few moments of fiddling with them he'd set them aside, grumbling to himself all the while. Each time he did this, Reina would raise her eyebrows, mutter something like "I told you we didn't need that," and be ignored. The Doctor avoided even touching the sponge as much as possible, treating the creature with a reverence they'd previously only seen him show towards his granddaughter.

For once, however, Ian found that it seemed the Doctor knew what he was doing. Unlike the irritated mumblings he made whilst trying to get the TARDIS in line, the old man sounded more and more satisfied as he progressed.

Eventually, it appeared that the pair of them had come to some sort of a conclusion. The Doctor straightened, holding the beaker with the sponge in one hand, and a small bottle with their own concoction in the other. He turned to Reina.

"You mentioned some sort of psychic shields," he said. "How do you, eh, activate them?"

"We use a gas we developed from the sponges," said Reina. "It's in the oxygen supply."

Susan looked queasy at the thought, but the Doctor's face cracked into a fiendish smile.

"Excellent," he said. "I'm going to need to use that."

Reina nodded. "The processing room isn't far from here," she said. "Just down this corridor."

And off they went, again, following corridors and passageways through the labyrinthine laboratories. Ian could only hope that one of the others remembered the way back, because he was quickly growing lost.

They finally ended up in a large room, with pipes running across the walls and a glass tank filled with a greenish gas sitting in the centre.

The Doctor set upon the tank, and Reina waved for the others to sit before returning to his side. The pair of them set about mixing the gas in the tank with the concoction in the bottle, drawing it from a tap on the side of the tank. They exchanged a glance with each other, and the Doctor sniffed the gas in his bottle, before turning to the sponge in the beaker, deep in concentration. There was a moment of silence, and then the old man burst into laughter.

"What is he doing?" Barbara asked.

"He's been trying to reverse the process of the psychic shields," said Reina quietly, turning back from helping the Doctor. "Instead of blocking the voices, he wants the gas to amplify them, make the communication go both ways. I think…" she looked at the Doctor, who was staring at the sponge intently.

"Yes, he is, isn't he?" said Susan. "Yes! I can hear it! He's talking to the sponge, isn't he?"

"Well," came a voice from the doorway, "we can't have that, can we?" Yandi stepped in, enraged. "I told you not to cause trouble, Reina."

"I couldn't stand idly by while you – while we committed atrocities," hissed Reina.

Yandi stepped forward, and they could see he had a gun clutched in his hand. "I'm afraid I can't let you do this," he hissed.

The Doctor's eyes scanned the room, taking everything in. He saw the gun in Yandi's hand, saw Ian and Barbara standing up to protect Susan, saw Reina crouch to lunge towards Yandi.

He did the only thing he could.

He stuck his bottle to the tap on the tank, and poured its contents in.

Their world exploded.