Part 1

Barbara had never seen anything quite like it. The TARDIS had landed in an enormous glass dome, with corridors connecting from all directions to what she could see through the glass was a cluster of other domes, some transparent and some opaque. Beyond this cluster, more clusters of domes were apparent, stretching as far as she could see.

This wasn't what really grabbed her attention, however. Regardless of how magnificent the domes stretching ahead of them appeared, they were nothing compared to the vast, colourful darkness surrounding them. Strange creatures swam past the dome, some appearing to be ordinary fish, and others a variety of shapes, glowing a spectrum of colours, looking for all the world like creatures from another planet. Which was, Barbara reminded herself, entirely within the realm of possibility.

Susan and Ian followed her out, and she heard Susan gasp behind her.

"Doctor, come and see this," Ian called.

They could hear the Doctor grumbling inside the blue box, still trying to get the scanner to work. Apparently conceding defeat – for now, at least - the Doctor strode out of the TARDIS. His irritated expression vanished, and eyes gleamed as he surveyed the domes ahead of them, appearing far more interested in them than the ocean around them.

"My word," he said. "I do believe we are underwater, in some form of city."

It seemed a reasonable deduction, but, gazing at the creatures outside the window, one question persisted in Barbara's mind.

"Are we on Earth?" she asked.

"What sort of a question is that?"

They jumped and turned. A young man, who must have only been in his twenties, was standing behind them, a look of bemusement on his face. His dress was very ordinary, even casual, but he had some sort of technology, like half a pair of glasses, hooked over his ear and in front of his eye.

"Are you from off-world?" he asked.

"That depends, my child," said the Doctor. "Where are we?"

The man gave them another confused look, and Ian coughed.

"Our ship isn't exactly… accurate," he ventured.

"Nonsense," snapped the Doctor. "The scanner needs repairs, that's all."

"You're on Earth," said the young man, eventually. "In Hydrea." He paused, before adding, almost as an afterthought, "At the bottom of the Atlantic." He looked over the TARDIS. "Is that your ship? Why does it look like a phone box?"

"Well, it's a disguise," said Susan.

"It's not a very good one," said the man. "How did you all fit in there?"

"Never you mind," said the Doctor, who had been growing more and more irritated with the conversation as it went on. "I must see more of this city. Where do you get your energy from, mm?"

"Here, actually," said the man. "We're a geothermal power plant. We generate energy from the Mid-Atlantic Ridge."

The gleam returned to the Doctor's eyes. "Fascinating," he said. "You must take me to see your laboratories."

"Do you think they might have some materials you can use to repair the scanner?" asked Ian.

"An excellent point," said the Doctor. "We shall see."

The man stared, stunned, his mouth opening and closing. "Sorry," he said, getting a handle on his vocal chords, "I'm just an intern, I don't actually…"

"Theodore!"

This time, it was the man's turn to jump and turn. A man and a woman, both wearing lab coats with their sleeves rolled up, came in from the corridor behind the TARDIS. The woman hurried over to the young man.

"Theodore," she repeated, "who are these people?"

"And why is there a phone booth parked in the observation dome?" asked the man.

"Sorry, sorry," said Theodore, rubbing his temples. "These are some… travellers. I think they're from off-world. They say their ship -" he indicated the TARDIS "- is broken down."

"We said nothing of the sort," said the Doctor.

Barbara stepped forwards. "We're terribly sorry," she said. "We'll move our ship as soon as we can. I'm Barbara, these are Ian, Susan and the Doctor."

"I'm Yandi, this is Reina," said the man. "I'm the head of this facility. I really must insist that you remove your… ship… immediately."

"Hmph," said the Doctor. "I'm afraid we can't move it just yet."

Ian and Barbara exchanged a glance. It was impossible to tell whether the old man was telling the truth or not, especially after the Dalek fiasco.

"Why not?" said Yandi.

"Because their ship is broken down?" Theodore volunteered.

"It is not, not broken," the Doctor repeated. "But the scanner needs to be repaired. We ought not land anywhere without the ah, the ability to see where we are. We were lucky this time, but who knows where we might end up next."

"I could have you arrested for causing an obstruction," Yandi warned.

"Really now," said Reina, under her breath.

"If you arrest us," said Ian, "nobody will be able to remove the ship."

"We could remove it forcibly," Yandi said.

Susan stifled a laugh. Yandi narrowed his eyes.

"Very well," he said. "We'll help you, if we can. What do you need?"

The Doctor's face broke into a wide smile. "Take me to your laboratories," he said. "If there is anything of use there, I will tell you."

Yandi waved his hand tiredly. "Very well," he said. "But we will only take two of you. The other two can wait in the cafeteria."

"Susan and I can wait," Barbara volunteered. "I'd rather sit and look at the ocean, anyway."

"Excellent. Chesterton, you will accompany me," said the Doctor. He turned to Yandi. "Lead the way."

"Theodore, take the women to the cafeteria," sighed Yandi. He jerked his thumb at the Doctor and Ian. "You two, follow us."

It was hard to imagine a better place to have your lunch. The fish weren't that colourful in themselves, that was true, but they made up for it with the lights that covered them from head to tail.

"It's called bioluminescence," Theodore was telling Susan.

"It's beautiful," said Susan, as an enormous eel with lights along its belly swam above them.

They sat around a large table, eating the sandwiches that Theodore had managed to procure for them while they waited. The cafeteria wasn't particularly busy, despite the fact that it was lunch time. It didn't seem that many people working here – but, though Barbara didn't know quite how far into the future they were, she supposed automation must have done away with the need for quite so many workers by now.

"It's nice to be somewhere as lovely as this, for once," Barbara commented.

"Oh, yes," said Susan. "I much prefer this to -" But before she could finish, she cut herself off with a yelp.

"Susan?" Barbara hurried to Susan's side, Theodore close behind. The girl was sitting, clutching her head in agony.

"Barbara, make it stop!"

"What is it, Susan?"

"Can't you hear them?" Susan held her temples, shaking. She looked into Barbara's eyes, her face painted with fear. "Horrible, horrible voices. Can't you feel it? Can't you feel the sadness?"