Part One

It was a beautiful day on a planet a hundred light years away. The suns shone brightly, the pink-tinted sky shimmered and brightly coloured birds hopped from tree to tree, cheeping loudly. Then all of a sudden, they flew away en mass and the sound of engines filled the air. A moment later, a blue London Police Box appeared out of nowhere. The door opened and three people stumbled out, surrounded by a cloud of vapour and coughing loudly.

"Doctor, you really MUST fix those mercury fluid links," Zoe Heriot complained, leaning against a tree and gulping in lungfuls of refreshing air. "That's the third time they've blown this week!"

The Doctor waved a handkerchief in the direction of the Police Box trying to disperse the vapour. "It's not my fault, the poor TARDIS still hasn't recovered from that volcanic eruption, she needs time to cool down" he complained.

"Well are we safe t' wait here?" Jamie, the Doctor's other companion asked.

"Oh yes, I should think so," the Doctor said cheerfully. He paused for a second. "Well… I'll check." He covered his mouth with the handkerchief and dove back into the open doors of the TARDIS. "Yes, it all seems fine," he shouted a moment later. "The atmosphere is similar to that of Earth. Pre-industrialisation. A smidge more nitrogen." He reappeared and closed the door behind him. "We'll need to find some mercury while the TARDIS cools down. Shall we have a look around?"

Before his companions could say anything, he strode off down a pathway. Zoe and Jamie shared amused looks and raced after him.

After a few minutes, the trees thinned, giving way to well maintained fields containing herds of unfamiliar crops and animals. The Doctor looked around hopefully and quickly found what he was looking for; an elderly, humanoid figure stood in one field, raking rows of small blue plants. He jumped the fence and jogged across the field, being careful not to damage the seedlings.

"Hello, I wonder if you could help us? I'm looking for mercury," he said with a beaming smile.

The old man ignored him and stared open-mouthed at Jamie and Zoe. The Doctor glanced back at them and grinned. They were a rather eye-opening sight, he in his tartan kilt and her in the silver, glittery catsuit. "Why's she's not in robes? And why's he wearing a skirt?"

Happily Jamie was too busy helping Zoe pull her foot out of mud to hear. "We're going to a fancy dress party. Do you know where I could get hold of some mercury? It's very important."

"The Science City," the stranger said immediately, still watching the Doctor's companions. "Go half a mile down the road there and you'll get to Kork. The Science City is at the end of the town square."

"Thank you." The Doctor nodded in thanks and picked his way carefully back to his companions. "Come along, you two. How long does it take to walk across a bit of mud?"

"It sticks," Zoe complained. "I'm ruining my shoes." She fluttered her eyelashes at Jamie. "Would you be an absolute darling and give me a piggyback back to the road?"

"No," the boy said heartlessly. "Serves ye right for wearing silly shoes."

Zoe made a face at his back and then stumbled. "Ow. Ow ow ow."

Jamie sighed, picked her up and carried her across the field to the side of the road. "There," he said, unceremoniously dropping her to the ground. "You can walk from here."

"Don't worry, it's not far," the Doctor said quickly. He strode off down the road, forcing his companions to stop bickering and hurry up.

"Where are we going?" Jamie asked, catching up and walking alongside him.

"The Science City in Kork, apparently."

"Oooh, Science City?" Zoe looked thrilled. "That sounds interesting."

"Sounds boring," Jamie muttered.

Zoe ignored him. "Can we look around when we get there, Doctor?"

"If they have no objections," the Doctor said with a shrug.

The three of them walked along in comfortable silence for the next few minutes. The road soon opened up onto a busier road, where vehicles hovered past, scattering dust in their wake. Buildings began to appear at the side of the road and people dressed in long, brightly decorated robes bustled past, occasionally turning to stare at the trio.

"Ah, that must be the place," the Doctor said, pointing across a busy market square to the impressive, pale stone building beyond. He led the way along the side of the square and climbed several steps to a heavy, wooden door. "Hello?" he called, pushing it open.

A man in dark red robes looked up from the desk he was sat behind. "Can I help you?"

"I hope so." The Doctor stepped inside. "I'm the Doctor. I was wondering if it would be possible to get some mercury?"

The man shrugged and picked up a telephone. "I see no reason why not, hang on a sec and I'll get someone down to-oh." He stopped dialling and stared at Zoe, who'd just stepped inside. "I'm afraid you'll have to wait outside, miss."

Zoe blinked in confusion. "Why?"

The man smiled kindly at her. "Girls aren't allowed in the Science City, you know that. You don't want to distract the scientists with your pretty face now, do you?"

The Doctor stepped back hastily and placed a warning hand on the girl's shoulder before she could react. "Zoe is a scientist herself," he said. "Surely you could make an exception?"

He shook his head. "I'm sorry, but it's the law. You should know that. She'll have to wait outside."

Zoe scowled nastily, spun around and stomped out of the building, muttering under her breath. "Oh dear. Jamie, you'd better go after her," the Doctor said, looking concerned.

Jamie nodded and ran out after the girl. He found her pacing back and forth at the bottom of the steps. "Are y' all right, lass?" he asked, sitting down.

"Girls not allowed?! What kind of backward, Neanderthal planet is this?!" Zoe snarled, giving the steps a kick. "Ow. Bunch of… of MEN."

Jamie tried not to laugh. Although he could understand why she was upset, her outrage was amusing to watch. "Aye, well, there's no' a whole lot we can do except wait for the Doctor."

"It's not fair," she muttered. "You don't care about science, but I bet they'd let you in, no questions asked, just because you have testicles."

"I have what?" he asked blankly. "Is it bad?"

Zoe ignored him. "Come on," she snapped, walking away towards the edge of the building.

"Where are we going?" Jamie asked, hurrying after her.

"We're going to look for a way in," Zoe said obstinately. "No girls allowed indeed!"

"I dinnae think that's a good idea," he said hastily as she walked along, gazing up at the walls.

"Scared?" she asked sweetly. "You can always go and loiter on the steps if you like." She stopped in front of a window and peeped inside. "Oh goody, empty. Give me a leg up in here and you can go."

"Och, I'm no going anywhere," Jamie said resolutely. "If you're going in, I'm coming with you. Someone needs t' keep an eye on you."

She smirked to herself and watched him pull a knife out of his sock and prise the window open. "After you," he said politely, standing aside.

She clambered up the wall and daintily jumped inside. The room was in darkness and it took several seconds for her eyes to adjust. When they did, she was delighted to see that the room was filled with shiny, tempting gadgets. "Oooh, Jamie, come and look!"

Jamie hauled himself over the windowsill and gazed around, unimpressed. "Looks like junk t' me. But dinnae touch!" he added sharply as she danced happily across the room and picked up something delicate-looking with wires sticking out of it.

"Isn't it marvellous?" she said dreamily, holding it aloft. "Oooh, and look at this! Wow, it's got a tachyon acceleration drive!"

Jamie ignored the technobabble and sat on a spinny chair. "We cannae stay long," he warned. "The Doctor will wonder where we are."

"All right, all right." She sighed and put on a strange metal helmet with lights on top. "Look, I'm a Dalek!"

Jamie couldn't help but laugh at her childish pleasure. "You look like no Dalek I ever saw!"

"What do you think it does?" she asked, picking up an identical helmet and examining it curiously.

"I dinnae know. Mebbe it's a hair dryer?" he suggested.

"Haha. Maybe it reads brain waves." She popped the other helmet on Jamie's head, just to annoy him. "No reading. You're clinically brain dead." She jumped aside as he swatted at her and accidentally stepped on a remote control that had fallen to the floor. "Oops-"

A blinding flash of blue light filled the room and suddenly she was staring at herself. "Huh?"

"Zoe?"

"Jamie?"

Zoe looked down at herself. Or himself, she wasn't quite sure which. "Well that was unexpected."

Jamie crossed her arms. Zoe's arms. "I told y' to no touch anything!"

"Goodness." Zoe grinned. "This feels weird. You should really wear underwear, you know."

"Zoe!"

"It's all right, I'll undo it." He took off the helmet, popped it on a filing cabinet, bent down in Jamie's body and picked up the remote control from beside her own foot. Then he paused. "Jamie?"

"What?" Jamie asked suspiciously.

"Can we stay like this for a little while? Just so I can have a look around?" he asked hopefully.

Jamie glared at him. "No! Come on now, turn us back. The Doctor'll be wondering where we are."

"Fine. Spoilsport," Zoe muttered, picking up the helmet again. Then he froze. "What's that?!"

Jamie grabbed his arm at the unmistakable sound of approaching footsteps. "Quick, hide!"

They dived behind the filing cabinet just in time. The door opened and a middle-aged man in bright green robes walked inside. He picked up the remote control and the helmet, and then began rummaging through piles of junk.

Zoe suddenly realised he must be looking for the other helmet, which was still on Jamie's head. As soon as the man had his back to them, he grabbed it and slid it out across the floor before Jamie could stop him.

Jamie moved forward to snatch it back and froze as the man turned back in their direction. He spotted the helmet, picked it up with a happy noise and strolled out.

Jamie and Zoe stayed where they were for a few seconds longer, then Jamie turned around and glared at Zoe. "What did y' give it t' him for?!" she hissed angrily.

"Did you want him to find us?!" Zoe snapped back. "No girls, remember. Who knows what they do to lawbreakers on this planet." He climbed to his feet and brushed down his kilt. "I'll follow him, you wait outside for the Doctor."

"I'm no' letting y' go wandering off in my body!" Jamie exclaimed. "I'm coming with you!"

"Fine. Do what you want." Zoe rolled his eyes and carefully opened the door. The corridor was deserted. He tiptoed quickly down it and peeped around a corner just in time to see the man in robes disappear around another corner. "Quick, this way," he hissed, waving Jamie forward.

Unfortunately, just as Jamie left her hiding place, a door opposite opened and several men stepped out. They stared blankly at her for a moment before one recovered his senses. "Oi! What are you doing here?!"

Jamie did the sensible thing and ran. After a moment's hesitation, the group of men ran after her.

Unfortunately for Zoe, they were all running away from him. He dithered for a moment, torn between following the man with the body-swapping helmets and making sure Jamie didn't fall down a flight of stairs and break her leg.

Following the mystery man won out in the end; Jamie was more than capable of taking care of herself. He turned away and jogged to the end of the corridor, looking hopefully in both directions. A flicker of green cloth caught his eye and he followed.

Meanwhile, Jamie was discovering that Zoe's body wasn't nearly as fit as her own. The legs were shorter, the boobs bounced uncomfortably and the lungs burned with effort. And the men were catching up.

She skidded around a corner and discovered that not only was Zoe's body not designed for running, neither were the very silly shoes on her feet. One heel collapsed underneath her and she fell flat on her face with a squeak of surprise.

Before she could recover, someone grabbed one of her arms and yanked her to her feet. She was spun around and found herself face to chest with four very annoyed looking men. "Er… I surrender?"

One of them grabbed her arm and hauled her to her feet. "What the blazes are you doing in here, you stupid girl?!"

"I wasnae doing anything," Jamie protested, pulling her arm free. "I was just…" She paused and resisted the urge to punch him in the teeth. "I got lost," she said finally, smiling sweetly at him. It was a tactic he'd seen girls use before to get out of trouble and it usually worked. "Cannae ye just show me the way out?"

"Oh we'll show you the way out all right," the one holding her arm snarled, giving her a shake. "Rhus, call that idiot on security and tell him to get the police."

"Police?!" Jamie began to struggle free, only for someone to grab her other arm. She protested loudly, but the men ignored her pleas and insults and frogmarched her away.