Rose had decided that this cave was never going to end.

Each time she had thought it might there was another twist, another turn or yet another tunnel that stretched on into the darkness. They had already travelled miles upon miles and they might have to go on the same distance again.

After the Doctor had led the way past the dismantled ship and along the narrowed path, their route had travelled downhill to an opening so low to the ground that it had to be crawled into on their hands and knees.

"Careful, Rose." The Doctor cautioned. "Just take it slowly."

"Why can't we just go around?" She questioned. "Take the other tunnel?"

"Because Peter and Stefan took that one."

"Exactly, they're in trouble. We have to help them."

"We are going to help them. But we can't do that if we get caught ourselves." He told her. "Look at the size of this tunnel. Either that baby creature dug it or they don't know it's here at all."

"So where does this lead to?"

"Right into their lair, I reckon. The same place they'll have taken their prisoners."

Rose flinched at the word.

The idea of Peter and Stefan being at the mercy of those creatures was too horrible to contemplate. They could already be dead, their throats torn out just like poor Tom Richardson's had been. But Rose refused to believe this was their fate. After all, they had gone through and how capable she knew the two soldiers to be, she had to remain hopeful that she would see them both again.

Especially seeing as there were so many questions that remained unanswered.

"Any idea yet, Doctor?" She began. "Do you know what kind of alien Peter is yet?"

"You mean species, Rose. Remember that alien doesn't always mean none human." He corrected her. "But to answer your question, I'm still narrowing down the options and… Oh! Look at this."

The tunnel had ended at a hole in the wall just a few meters above the ground of a small cave.

But this space was not some empty and dank gap in the rocks. It had been made into a liveable home and solved the mystery as to where at least some of the creature's ship had ended up.

It was like a crude parallel to the base Peter's team had set up. A large table and a bench was running alongside it, both of them made of metal and quite simple and functional in their design. Panels from the ship's hull had been used to line the walls, covering the rocks. Alternatively, some pieces had been flipped onto their sides so that they could be used as shelves.

Jumping down from the hole, something sat upon the table caught Rose's eye.

Shaped like a cone, the metal object was about the length of her forearm. Part of it had been cut away, however, so that a spiralling pattern curved down its side.

She was shocked to realise that it might just be an ornament.

These creatures were intelligent life-forms and not the mere monsters that they appeared to be. Picking it up and examining it for a moment as her fingers brushed over the grooves, she carefully set it down again and took another look around the cave.

"They've been living here." She said. "They're stuck here so they've made it their home. Decorated it and everything like a proper house."

"Must have been desperate to tear apart their ship." The Doctor mused. "But this can all wait for now. We need to find our missing soldiers. If anything's happened to them…"

"They'll be okay, Doctor," Rose responded. "They have to be."

The Doctor nodded at this but his brow was still creased into a frown.

"Even so, they're UNIT and that makes them my responsibility." He told her seriously. "Especially when they're in danger."

"Try telling them that." She reminded him. "Peter wanted rid of us pretty quickly."

"He just doesn't trust us yet. Hopefully, we're not too late to remedy that."

Walking out of the cave, they found themselves standing in a hallway of sorts. Crude doorways led off into other small caves and tunnels that had been converted into more rooms and corridors.

But perhaps even stranger than finding the ornament was the discovery of lights that had been strung up along the walls.

"I thought they couldn't see?" Rose quizzed upon spotting them. "Why bother putting lights up?"

"In bat form, they won't be able to see." The Doctor explained. "But remember that I said that they had the biological traits of a metamorphic race?"

Rose remembered Peter explaining to them what that meant.

"They can change what they look like." She remembered. "Become something else."

"They must take on traits of their victims too. Blend in on whichever world they land on." He continued. "But why, though? What do they get from doing that?"

"Doctor, how are we going to find them?" Rose questioned. "There's no way of telling which way they went."

"Yes, we could have done with Peter's super-duper hearing right now, couldn't we?" He said, smiling. "But this works just as well."

Pulling the sonic screwdriver out of his pocket, he began to gently wave it up and down through the air as the little metal tube started to buzz and whirr softly. Quickly, the movements concentrated and the Time Lord soon had it specifically pointed toward one of the tunnels.

"What are you doing?" Rose asked, seeing him smile. "What is it?"

Without explaining his actions, he took hold of her hand and pulled her after him.

"Come on. This way."

Peter had no idea how long he had been unconscious for. He doubted that it couldn't have been more than a couple of minutes even though it could have just as easily been hours. Sick and dizzy from having the blood sucked out of him, he tentatively lifted his head to see where the deadly alien siblings holding him captive had gone.

That was when he saw himself, stood at the foot of the table along with the stolen form of Tom Richardson.

It was the most peculiar thing to see his own face outside of the confines of a photograph or a mirrored reflection of a mirror and it took Peter what felt like an eternity to accept what he was seeing.

There was not one imperfection in Karugon's new form.

He was looking at a twenty-three-year-old man with messy black hair, sterling grey eyes and a face framed by a strong jawline. The alien was even dressed identically to him, the black clothing dishevelled in the same way as his own was.

Peter doubted that his mother would have been able to tell them apart.

"Quite unnerving, isn't it?"

Kalagan had spotted him staring and smiled triumphantly as he walked over to him. Stooping down so that he was now level with Peter's head, he looked somewhat amused as he relished in his captive's confusion.

"I've seen stranger." Peter bit back. "Far stranger."

"I'm sure that you have." He replied. "Coming from a world beyond this one. You must have seen all manner of things."

"Where is your world, lieutenant?" Karugon quickly added. "Where are you from?"

Peter felt his blood run cold.

Having his voice stolen too was bad enough. But there was no questioning now that the creatures were getting close to finding out what he was. Then it dawned upon Peter what they had told him. If these creatures took on the biological traits of their prey as well as their appearance, did that mean that Karugon now had the ability to…

"If you're me, can you..." He started to say. "You can't possibly..."

But Peter shut himself up the moment he saw Karugon frown at him. The alien did not yet fully understand the body which he had inherited and it was fortunate that he hadn't already discovered for himself its abilities.

Fortunately, Kalagan seemed to have no interest in discussing the matter further.

"Never mind that." He snapped. "We have more pressing matters to attend to."

Peter recoiled in disgust as he came nose to nose with the dead man's face. He could smell the repulsive stench of blood and human flesh still lodged between his teeth, and nearly gagged on it.

"What do you want from me?" He questioned through gritted teeth. "You already have my face, my lifespan. Isn't that what you wanted?"

"Oh but we still know nothing about you," Kalagan told him. "I fear that your presence here and my missing family are connected."

Peter said nothing about this.

If they learned that their three relatives were dead because of him and his team then they would not hesitate in killing both him and Stefan as punishment for it.

"You want to know about me?" He asked, deliberately casual. "Why? I'm nobody special."

It was another lesson to be learnt about surviving an interrogation. Treating the whole thing as a normal conversation would relax the situation somewhat and make it less unpredictable. At the very least would buy him some more time.

"I told you that we retain our ability to sniff out our prey," Kalagan told him. "We can also use it to detect one another once transformed into the natives of any world we come to. Regardless of what we end up looking like."

"Big deal." Peter retorted. "You're not the only ones with a good nose."

"But imagine my surprise, lieutenant." The alien continued. "When I caught the scent of a Time Lord upon you."

It was a smell that reminded Peter of a dusty old book. A large encyclopaedia perhaps that had lived in a library all of its life. It was perfectly suited to a man who had so much knowledge tucked away inside his head.

The Doctor was not here, of course. But there was a faint trace of his presence lingering in the air that was barely detectable even with a nose as sharp as Peter's was.

Why they were interested in him was obvious.

These creatures were all about gaining as long a life as possible. A human could normally only live eighty or ninety years with any luck. So far as he knew, and if they didn't kill him first, Peter could expect the same amount of time. But the Doctor was ancient. He did not die but continued living on at the cost of changing his shape.

"He regenerates," Peter stated. "You'd live forever if you became him."

"Exactly, so tell me where I can find him?"

It was like something out of a James Bond film, the part when the spy and the villain came face to face. But Peter was not a spy, and he knew that no movie could ever reflect the level of the very real danger he was now in.

"Do you see him here with us?" He countered. "He's not here."

"Lieutenant, do not play the fool because it does not suit you." Kalagan hissed. "Tell me where he is before I have to hurt you."

Peter felt a spark of anger trigger within. He did not like being threatened in such a way and it was frustrating that he could do very little about it right now. If he was stubborn and refused to cooperate then he would almost certainly be killed. But he couldn't let these creatures become immortal, not when there was no telling what would happen as a result.

Even if he knew where the Doctor had gone, he couldn't give him up. There were too many consequences to comprehend. Besides, the game wasn't over yet. He could get himself out of this or he would die trying.

"You're right, Kalagan. I'm no fool." He growled back. "Neither are you. Which is why we both know that I am not going to give up the one piece of information that's going to keep me alive."

Kalagan smiled as though this was a fair point to be made.

"Very well, how about an exchange?" He proposed. "We'll let you both go unharmed and we'll leave this world's inhabitants alone if you give us the Time Lord's location. Does that sound fair?"

Peter knew when he was being lied too.

Hearing the shift of pace in a person's heartbeat as they tried to deceive him was just one way of knowing. But it wasn't required this time, not when such an empty promise of a fair exchange was being made by aliens who had already proven just how merciless they were.

There was no chance of him being set free even if he betrayed the Doctor.

But Peter could still play along. He could acquire some useful information and try to delay the inevitable moment they decided he wasn't worth keeping alive anymore. Stefan might wake up and get free. He might be able to get back to the team and warn them.

Might... It was a strong word right now. But it and a little hope were all Peter had.

"Before I do tell you what I know. I think it's only a courtesy that you tell me who I'm dealing with here." He said. "What species are you? Where are you from and how did you get here?"

Kalagan contemplated this for a moment before nodding in agreement.

"Our planet is nothing but a breeding ground. We do not have a long enough lifespan to waste our time making a home there." He dismissed. "We wander the known universe looking for new forms and lives. We rarely stay anywhere long, no more than a few decades at a time."

"You have a ship?"

"Of course we do. You and your companion were found lying right next to it."

Peter could not recall seeing one and knew he must have already been unconscious long before he'd come anywhere near it. There had been no mention of a spaceship in the briefing either, and no indications whatsoever that anything had entered Earth's atmosphere.

It was one of the first rules UNIT had taught him.

Aliens did not always come in a great big spaceship in their thousands and threaten the entire the world. Some would arrive in unexpected ways and pose a danger to the Earth that might not even be comprehendible.

"But how did you get here? We would have noticed a crash landing, so how the hell did you end up below ground?"

Kalagan's face suddenly fell as he shook his head sadly.

"This is something for which I have no answer." He told him. "Our ship was in flight when there was a flash of light. It completely enveloped us. There was this sound… It was though we had flown into a storm that had appeared from nowhere."

"Lightning in space?" Peter wondered. "Is that even possible?"

"Your guess is as good as mine. But the next thing we were aware of was that the darkness of space had been replaced by this cave."

"So you moved in here? Put up lights and unloaded everything?"

"We reverted to our true forms and spent weeks discovering this place. But with our lifespans so short, we knew we had to find a way out to the surface."

"Why didn't you just go straight up?"

Kalagan explained himself by pointing up at the roof of the cave.

"Listen to what is above us."

Concentrating, it took Peter a moment to navigate past the layers of rock and dirt that separated them from the outside world. But he could just about hear the muffled sound of running water.

A flash of an image crossed his mind. It was the map of the immediate area they had been searching and less than twelve klicks away from his base was a large river. He'd forgotten the name of it but they must be right underneath it given the direction they had travelled in.

"All that water… If you dig straight up then this whole place will flood."

This was the first piece of useful information. But it was nothing that could help him right now.

"Which is why we were in need of finding new forms. We cannot stay here forever and this world must have means in which we can use to leave and continue onto more desirable civilisations."

"Well sorry to disappoint you," Peter told him. "But Earth doesn't have intergalactic ships. Space travel only extends as far as the Moon. Besides, you've killed someone. Tom Richardson was innocent and didn't have to die. You're also still holding another human hostage. So until she is released UNIT and I won't let you do anything to help you."

"Is that so?" The alien questioned. "How disappointing."

At once Peter knew that he had opened up far too quickly and with too much of an aggressive tone. It was just plain foolish to antagonise his captors now and it wouldn't end well if they turned on him. But if he showed any fear or weakness then they equally take advantage of that too.

"Who is this UNIT?" Karugon questioned from the other side of the table. "Are they this world's leader?"

"No, but they're more than ready to step in if you do not surrender yourselves. They've faced far worse than you and won't hesitate to use lethal force to end you."

"By consulting with a Time Lord?" Kalagan chuckled. "Such a dangerous species. But useful to us right now."

The conversation had come around full circle and back to what the creatures wanted to talk about. Peter suddenly felt trapped and unable to steer away from it again.

"I know how incredibly annoying they can be." He muttered. "But if you want him then you'll have to find him yourself."

Kalagan's brow creased into an irritable frown.

"What of our agreement?" He asked. "You asked us for information and we gave it to you."

"You also said you would let us go and leave the humans alone." Peter dared to remark. "Plus I don't recall us shaking on it. My hands are kind of tied down if you haven't noticed."

Kalagan's retaliation was swift and decisive. Seizing hold of Peter by the throat, he clamped down hard on his windpipe and began to squeeze.

"You think you are in any position to withhold information from us!?" He growled. "I warned you what would happen if you did not tell me where the Time Lord is!"

Black spots were beginning to cross in front of Peter's eyes. He couldn't inhale but was instead being forced to gasp out air, making him lose it even more quickly.

Suddenly he was released and the world came back into focus. Breathing in a deep lungful was painful but so welcoming that Peter didn't care what happened next. All he could focus on was getting as much oxygen back into his body as was possible.

Kalagan's expression was cold and harsh. He was leaning over him, ready to make him suffer until he got what he wanted out of him.

"You have wasted enough of my time." He snarled. "I do not care what you are or who you work for. We are untouchable and you will tell us where the Time Lord is or I will kill you!"

"W-will you now? Then go right ahead."

Stumbling at first, Peter suddenly felt braver as he lifted his head to meet with the alien's murderous gaze.

The sudden surge of confidence worked as he saw Kalagan debate over what action to take next. If the creature killed Peter before he obtained the Doctor's location then there was a high possibility that they would never find him. So they couldn't do anything too bad to him just yet.

"You do not fear death?"

"I know an empty threat when I hear one." Peter countered. "Kill me and you can say goodbye to any chance of finding that Time Lord."

"Then I will kill your companion first." Kalagan spat. "His death will be on your conscious."

"If you touch a hair on his head then you might as well kill me because I won't say another word. Besides, if we're not back by dawn then a whole army of soldiers will come in after us. They will find you and slaughter your entire family. No mercy shown whatsoever, Kalagan."

Peter's head snapped back as Kalagan slapped him across the face again.

"I can hurt you." He hissed. "I can make it painful."

With his face stinging from the blow, Peter rolled his tongue over his teeth to check that they were all still there before he turned back to face him.

"Try me." He dared. "Because I might fear death and whatever you're going to do to me. But no matter what happens next I do not betray people, and I will endure a hell of a lot before you come to the conclusion that killing me is going to be your only option!"

Peter didn't realise until now that he was shaking in his anger. His heart was hammering against his chest and he had begun to shout. Even Kalagan was taken aback by this and stared at him, seeming impressed by his resilience.

But then a cruel smile twisted across his lips.

"Karugon. Break something."

Peter barely had time to register the words before he heard the crunching sound of a bone being snapped clean in two. The unbearable pain which followed a split second later as the message reached his brain told Peter that it was his own leg that had been broken.

The breath was completely knocked out of him, and it was for this reason alone that he didn't scream. But he wished that he could have. Perhaps it might have aided in venting some of the agony out. Instead, Peter's head flew up, his eyes wide with sickening shock. He saw Karugon still leaning on his shattered leg, the alien having broken it with a single devastating twist.

His stolen features twisted into a gleeful expression as he gazed with delight at Peter. Pointing at him excitedly, he looked over at his brother.

"Kalagan, look at that!" He gasped. "Can I do that too?"

"Yes, I think you will be able to. Whatever that is, of course." Kalagan laughed back. "You are full of surprises, lieutenant."

"Go to hell!" Peter hissed in-between the short, shallow gasps he was able to wheeze out. "Ah!"

He shut his eyes, hiding them from the world. The pain from his leg had not lessened and Karugon had not lifted his hand away. Even this small amount of pressure was almost too much to bear. But just a few minutes more, Peter thought. Just a few minutes more and it would all be over….

"Perhaps now you will tell me where the Time Lord is?"

Fury overtook him so quickly that Peter did not even realise what he had done until he'd done it. Blindly throwing his head forward, he knew that he had found his target when a fresh burst of throbbing pain exploded across his forehead as his skull collided with Kalagan's face.

The alien reeled back, clutching a bloodied nose.

"Why you little…" He hissed. "How dare you!?"

"You're going to have to do a lot better, Kalagan!" Peter yelled back. "Because I am not about to go down without a fight!"

He had barely got the words out of his mouth before Kalagan stormed over and took hold of him by the throat again, this time slamming his head down onto the table.

Peter blacked out momentarily as his skull rebounded off the solid metal surface. Feeling it roll to the side as it came to a rest, he fought against the swimming dizziness and managed to open up his eyes.

That was when he saw the Doctor.