Mark and Kate had been together for over two year. It was the summer holidays and they were backpacking around Europe by train and ferry. They had worked their way down through France and Italy and were now in Greece, travelling from island to island. On this particular day they had hired bikes and were cycling along a deserted road to a beach at the far end of the island.

When they'd picked up the bikes, they'd also been given a small cable bike lock. The cable was covered in red plastic and locked by inserting one end of the cable into the lock at the other end. Kate had put the lock into the basket on the front of her bike and had stuffed the key into the back pocket of her shorts.

Mark cycled behind Kate watching her struggle up the almost endless hills. She had a nice figure, but was quite petite and slim, weighing only 7 stone. Her skin was pale and glistened with sweat from their exertions. She was wearing a baggy blue t-shirt and cream coloured shorts, which while not exactly tight, did show off her cute butt. Mark focused mainly on this as she stood up on the pedals and fought her way up the next hill.

At the top, they stopped to look down over the cliffs to the beach beyond. As Mark picked up a bottle of water from Kate's basket he saw the bike chain.

"I doubt this would take much to break," he observed as he picked it up.

Kate had little interest in the lock, but watched as Mark put it around his neck and locked it in place.

"What are you doing?" Kate asked surprised.

Tom's heart was thumping like mad, but he kept his cool as he tugged at the lock just to confirm to himself that he couldn't either break the lock or slip it over his head.

"He's the key, you fool," Kate said as she slipped her hand into the back pocket of her shorts.

"You keep it."

"But how will you get it off?" Kate asked somewhat confused.

Mark thought about how he could best explain this to his naive girlfriend.

"I like having this locked around my neck and you having the key."

"Why?"

"I don't know, it's just kinda sexy."

"Sexy, why?"

Why was a difficult question to answer, it just was.

"But what if I lose the key?" Kate asked, "thinking through the practicalities."

That would be sexy too, Mark thought but said nothing and just shrugged his shoulders.

"You're crazy, but OK, if that's what you want," Kate said as she pushed off and freewheeled down the next hill.

Mark pulled at the chain as hard as he could and smiled as it stayed firmly locked. Mark was so much stronger than her, more confident, more social and academically a higher achiever. He always held all the power in their relationship. Kate, on the other hand, was also permanently worried that he would leave her, which only added to her subservience. Now for once, she had some power over him and it felt really nice.

He caught up with Kate and followed her down to the beach. They dismounted and lent their bikes up against a steel bike rack that was set into the concrete road. Mark sat on the ground, leaning against the bike rack and drank some more water. As Kate walked over to him, he reached her waist and pulled her closer to him so that she stumbled forward and the front of her shorts pushed up against his face.

"Stop it," Kate giggled as she tried to pull away.

"Will you lock me to the bike rack," Mark asked running his hands up the inside of her shorts.

"What?" Why?"

"Just because. There's no one here other than us."

"But, you wouldn't be able to get away."

"I know."

"So why do you want to do it?"

"I just want to see what it feels like," Mark continued, getting more and more desperate for her to chain him up.

Kate was reluctant, but she'd do anything to please him and so took the key and unlocked the bike lock and then relocked it around both the rack and her boyfriend's neck.

"Like that?" she asked.

She looked down and saw Mark pulling at the lock, breathing quickly.

"Do you want to keep the key?" she asked innocently.

Mark just took the key from her and stuffed it into the back pocket of her shorts.

Kate stood there looking a little confused. "Is this what you wanted?"

"Yes, this is nice," he replied, grossly understating his view of the situation.

"OK."

"Why don't you take a look at the beach?" Mark suggested.

"Oh OK," Kate replied as she started to get the message that he wanted her to leave him there.

Kate walked to the beach and sat down on the sand with her knees drawn up in front of her. This was all a bit strange, why did he want her to lock him up? She leant over to one side, retrieved the key from her pocket and played with it in her hand. He really was helpless. If she didn't unlock him he'd probably be stuck there all night.

Kate started to smile, she didn't know why but she found that funny. It felt nice to think that he was dependent on her for something. There was no chance of him cheating on her while she had him locked up. In fact for once she felt confident in their relationship. So what if her worst nightmare came true and he told her that he'd found someone else. Fine, she'd just leave him chained up and whoever the tart was would have to settle for a boyfriend who was chained to a bike rack. She realised that these thought were starting to turn her on.

Once Kate was out of sight, Mark couldn't see or hear anyone, it was as if he was completely alone. He pulled at the bike chain and at the bike rack, trying everything he could to get free, but to his delight, everything stayed resolutely in place. He was completely stuck, he was Kate's prisoner. 'Kate's prisoner', he smiled again to himself.

If Kate left without unlocking him and if no one else passed by, which seemed quite possible in this deserted place, he would eventually die of thirst, still chained to the rack. The thought was both scary and exciting.

That night Kate and Mark were lying on the bed in their cheap hotel room, with the noisy, but ineffective, fan whirling above them. They had returned the bikes, but had bought two more bike locks. One was now locked around Tom's neck and the second was locked through the first lock and around the frame of the old metal bed.

Mark was lying on his back and Kate, who was already asleep, was lying on her side cuddling up to him. Due to the heat, they had pulled off the sheets and Mark was admiring Kate's slim body. He ran his hand down her back and around her white cotton knickers which were the only clothes she was wearing. He had specifically asked her to keep the keys to the bike locks inside her panties and he could now feel where the keys had come to rest between her cheeks. As he touched her, she turned over and smiled in her sleep.

Kate was now lying on her back with the keys pinned beneath her butt. Mark tried to slide his hand underneath her butt see if he could reach the keys, but there was no way he could reach them without waking her up.

She opened her big eyes sleepily, "You still awake?"

"Yes, you still got the keys?"

She rolled on to her side and felt her butt. "Yes. You really are crazy you know," she smiled.

"You really are sexy."

Kate giggled.

"Anyway, I can always get the keys if I wanted to," Mark bated her as he pulled her on top of him and ran his hands around her narrow hips.

"Not if I throw them away," Kate smiled, accepting the challenge.

Mark watched as Kate retrieved the keys, took careful aim and threw them across the hotel room and into the en suite bathroom where they landed on her shorts. Now he really was stuck. Even if he was able to pull the bed across the room, he certainly couldn't pull it into the bathroom. He was now dependant on his girlfriend to free him.

"Can't get them now," she giggled as she sat on his chest and playfully pinned his hands down. Mark smiled and with one quick movement freed his hands, rolled her over and lay down on top of her, now pinning her hands firmly to the bed. Kate squirmed with delight as she tried in vain to free herself, before giving up and kissing him.

"I think you've forgotten about this," Kate purred as she finally got one hand free and tugged on the bike chains which locked Mark to the bed, "without my help you're not going anywhere."

"And you're not going anywhere with me on top of you," Mark replied.

"You'll fall asleep soon," Kate giggled, "But that chain won't be going anywhere, you're locked in that forever."

"You win," he conceded, completely blown away by this increasingly dominant woman.

Kate cuddled up to him again.

"What about the keys?" he asked.

"In the morning," she whispered as she nuzzled into his neck.

Two hours later Mark woke up, his mouth parched. He eased himself out from his cuddling girlfriend and tried to stand up. He didn't get far before the chain pulled him back. Kate was fast asleep and he knew she didn't like to be woken.

"Katey," he whispered, nudging her slightly.

She opened her big eyes and blinked.

"Sorry, I need a drink, can you let me out."

A smile crossed her face as she remembered his predicament. She sat up and walked to the table and picked up a bottle of water.

"Just the keys will be fine."

"Well maybe that's not an option," she purred, watching the reaction in Tom's eyes as she returned with the bottle and sat down astride Tom's chest and held the bottle to his lips.

"Are you holding me prisoner against my will?" Mark asked.

"Prisoner, yes. Against your will, I doubt it," she purred.

"But you are refusing to let me out?"

"Yes," Kate replied.

They continued to travel through Greece, then back through Italy and the Alps. Every night Kate chained Mark to the bed, or occasionally to the radiator which meant he had to sleep on the floor. During the day Kate would also lock him up wherever she could do so without anyone seeing. During this time, sex was fantastic.

On one occasion they arrived at a station and left their backpacks in left luggage. As Kate closed the locker and typed in a code to lock the steel door, she saw Mark looking at her.

"I know what you're thinking," she smiled.

"Just for half an hour while you have a coffee?"

She smiled and opened the next locker. "In you go then."

"Just half an hour?" he checked.

"If you want me to lock you in here, then I'll let you out only when I'm ready."

"Yes, but we only have one day in this city and there are things I want to see."

"Your choice," she smiled.

Mark looked at her, trying to make out what she was thinking. He checked that no one was looking and then bent down and climbed in.

"Not too long?" he asked from inside the steel box.

"Do you want me to close this door or not?"

She looked so sexy in her denim shorts that she had made herself by cutting slightly too much off from an old pair of jeans. He nodded.

Kate closed the door, typed in six numbers and the door clicked and locked.

"I'd better tell you the combination, just in case," Kate called through the small vent holes in the door.

"Why, you going to leave me here?"

"No, just in case I get lost or forget the number, you know what my memory is like."

"Don't tell me, I'll take the risk," Mark replied.

Inside the steel locker it was dark and a little daunting. Perhaps he should have let her tell him the combination, that way any passerby could free him. But he also liked the idea that only she knew the number to open the box. As the hours passed, the idea turned him on more and more.

The day after spending over three hours in the locker, Mark had bought a length of chain and a combination padlock. The padlock had 4 wheels and 10,000 possible combinations. They were now in Switzerland and had just walked up a valley to the peek at the far end. It was late afternoon and they were making their way back down and as far as Mark could tell they were the last people to come down from the mountain.

"Katey, can we play one more game?"

"Sure, what do you have in mind?"

Mark took the combination padlock from his backpack and handed it to Kate. "Choose a new combination," he said.

As she worked, Mark took the chain and wrapped it around his own ankles. "OK, lock me up."

"How are you going to get down the mountain with your feet chained together?"

"I'll crack your code,"

"That will take you ages." she replied amazed.

"Maybe, maybe not."

"OK, if that's what you want," Kate smiled as she knelt down and threaded the lock through the chain, tightly securing Tom's ankles. She closed the lock and turned the wheels. "OK, that's you locked up," she said matter-of-factly.

Mark pulled her towards him and started to kiss her while unbuttoning her denim shorts. Very soon they were making out on the grass. When they had finished Kate redressed and stood up.

"Well I guess I'll see you later," she purred as she stood with one foot either side of where he lay in the grass.

"What, oh yes," the sex made him forget his earlier request and the day was now wearing on, "maybe we can play that game another time."

"Too late now," Kate giggled as she playfully tapped his face with her trainer.

"No really, another time."

Kate started to walk away and had to pull free as Mark was holding her leg. Mark climbed to his feet and jumped over to where she was, but she quickly skipped over a rock and was again out of reach.

"Kate, really let's do this another time."

Whether Kate was misinterpreted this as just part of the game, or not, she blew him a kiss and walked off down the hill. Mark watched her go.

This had been his idea. He sat down and started to turn the dials. However it was a good quality lock and each number had to be tried individually, he couldn't just spin the wheel around. He started with Kate's birthday, the date that they met and all the other obvious combinations, but nothing worked. He then tried all of those numbers backwards, but again nothing worked. What would she have chosen?

Kate was now a dot on the valley floor and well out of calling range. It was no good, he couldn't work out what she had picked, he would have to work his way methodically through all the possible combinations.

He started at 0-0-0-0 and worked upwards. By the time he'd reached 5-0-0-0 it was getting dark. By the time he'd reach 9-9-9-9 it was dark and he knew he'd missed the right combination and would have to start again. He put on his jacket and wrapped the emergency blanket around him and started again.

He couldn't help but think of Kate back in the hotel. What would she be doing? What would she be wearing? He hadn't wanted to do this, but he gave in and pulled out his phone and called Kate.

"You still haven't cracked the code?" she exclaimed.

"Not yet."

"Is it cold?"

"Not too bad."

"Good."

"So what's the number?" Mark bit his lip, he really wanted to get free himself, after all it was his idea and his lock.

"Do you really want to know?" Kate giggled.

"Yes."

"Well I'm not going to tell you."

"What!" Mark hadn't foreseen this. He had introduced Kate to some bondage games, but on his terms. He wanted to be tied up, but yet he still assumed he would have ultimate control. It seemed that he was now starting to lose control. Part of him felt like screaming at Kate, but part of him felt excited about the new developments.

"Good luck, sweetheart," Kate purred before hanging up and turning off her phone.

It was now very dark and cold and Tom's fingers were both sore and numb. The lock finally popped open, over four hours after Kate had locked it with the number 8-5-6-2. Why had she chosen that number? How could he have guessed that?

Using what little moonlight there was, Mark slowly picked his way down to the village. It was almost 9pm when he knocked on their hotel door. Kate was wearing her little black dress, fishnet tights and high heels, she looked fantastic.

"You finally solved my code," she purred.

"Yes, I didn't realise quite how long it'd take."

"So I see."

Mark showered and then they caught last orders in the restaurant. They then returned to the hotel room. Kate opened her bedside drawer and held up another combination padlock, but this time with five dials. "It took you four hours to solve four dials, so five dials should be.... umm forty hours."

"Put the mattress on the floor by the radiator and lie down," Kate ordered.

Mark followed her instructions and lay down with his neck next to the heavy wrought iron.

"Don't want you to peek now," Kate purred as she lifted her dress and placed her little butt down on his face. She then wrapped the chain around the radiator and his neck and slipped the padlock in place.

She turned the top dial of the padlock. "10 combinations to choose from, even you could work that out."

She turned the second dial,"Umm, maybe 2 minutes to get out of that."

She turned the third dial, "Now we're talking 20 minutes, still quite achievable."

She turned the fourth dial, "Now around 3 hours or longer given your track record. Either way, you should be able to get out before morning."

She turned the fifth and final dial, "Now up to 30 hours or let's say a couple of days. That's a long time to lay chained to a radiator.

Kate slid off his face and Mark watched as she continued to turn all of the dials. Kate's fishnet covered thighs, together with the clicking of the lock was turning him on and he pulled her towards him and they quickly started to kiss and make out. When they were finished Kate lay next to him and quickly fell asleep.

Mark looked down at her black silk underwear that covered her perfect little butt, but knew that it no longer contained the key to release him. He moved up her body and gently brushed her hair, knowing that somewhere inside her head was the secret number, the number that only she knew, the number without which he could never get free.