Almost immediately after setting off from their rest stop, the way through the cave become both obvious and problematic.

The tunnel they were now facing was narrow and lined with large boulders, making it difficult to navigate as it zigged and zagged at odd intervals.

"I don't like this." Stefan muttered to Peter. "Perfect place for an ambush."

Peter nodded in agreement.

"Eyes on. Stay alert." He replied. "Cover the rear, keep it tight."

"Rodger that."

As the lieutenant led the way down the tunnel, Rose was beginning to feel trapped in as she and the Doctor were wedged in between him and Stefan.

There was room enough not to be forced into walking in single file, but the serious faces of the two soldiers was making her think twice about moving out of line.

Rose shuddered at the thought of one of those creatures making an appearance down here.

"Hang on." Peter said suddenly, holding a closed fist up into the air as he immediately came to a stop. "Something's not right."

"What is it?" Rose asked him.

He didn't answer her.

Staring down the length of the tunnel, his whole body had tensed up at whatever had caught his attention.

Sensing the danger through his friend, Stefan silently advanced past the Doctor and Rose as he brought up his crossbow and pointed in the direction Peter was looking in.

"Where is it?" He whispered. "Tell me where to aim."

But there was nothing to aim at.

Peter hadn't lifted up his shotgun, and there seemed to be no indication whatsoever that anything was even remotely close to them.

Still, the lieutenant had proven twice already that he had a knack for detecting unseen dangers.

"Is it another creature?" Rose asked. "Or is it -"

"Shush!" He hissed at her. "These things hunt by sound, remember?"

They all fell silent again, but no new noises arose to indicate that another creature was nearby.

"Are you sure there's one of them around?" The Doctor asked. "You know, sound can echo off the walls and -"

Peter suddenly rounded on him, angry for the continued interruption.

"Yes, I am sure, Doctor." He snapped back. "Now shut up and let me -"

It had been just a momentarily of lapsed concentration, but that was all the creature needed as it pounced.

There was a gap in the rocks just above them, one no bigger than a football, and it was from there that the tiny clump of black skin and claws had jumped down from.

Stefan screamed as it landed on his chest.

"Ah!" He cried out as clung onto him. "Get it off!"

Still holding onto the crossbow, he accidentally squeezed the trigger and sent the bolt flying out of it.

If Peter hadn't immediately dropped to the ground it would have hit him.

Hearing it whistle over his head with just centimetres to spare, there was a very loud splintering crack as it struck a rock behind him and broke clean in two from the impact.

Jumping to his feet, Peter wasted no time in rushing over to help his friend.

The tiny creature was just as lethal as its fully grown counterpart, and already had a full set of miniature fangs and claws.

It took both of them to pry it off of Stefan's neck, and even then it was continuing to try and bite and slice into any nearby flesh it could grab onto.

"Oh! Why you little…" Peter hissed as it went for his left hand. "Right, that is it!"

With a powerful throw of his arm he managed to toss it away, sending crashing against a boulder where it fell to the floor a moment later.

Only slightly dazed, however, it soon regained its footing and was preparing once again to attack them.

BANG!

In such enclose surroundings Peter's shotgun sounded like a cannon blast as he fired a single, decisive bullet.

There was hardly anything left of the infant creature, just a few scraps of black leathery skin scattered on the ground and stuck up against the now blood splattered rocks.

Rose felt sick.

"No!" The Doctor whinnied, frowning. "You didn't have to kill it!"

Breathless from the struggle, Peter did not respond.

But the soldier's venomous look was enough to make his feelings on the matter perfectly clear. Both he and Stefan were bleeding, Peter from his bitten hand and the sergeant from two large gashes on his neck.

Until that point, Rose hadn't realised that she had been holding in her breath the entire time and she finally allowed herself to exhale.

"So much for all babies being cute." She breathed. "That thing was nasty."

"You're telling me." Stefan hissed. "Ow, damn that hurts."

Leaning against the rocky wall, he tentatively touched the deep cuts with his fingertips, wincing sharply as he did so.

"You still didn't have to kill it." The Doctor continued to protest, frowning at Peter. "It was just a baby."

"Are you insane!?" He growled back. "Have you see what those things become? If they're breeding, we have a major containment problem on our hands."

Without waiting for a response, he turned his attention back to Stefan.

"I don't think it's that deep." He told him as Peter made a quick examination of his neck. "What about you?"

"Yeah, I'm good." He replied. "It's just a scratch."

Not even bothering to look down at his hand, Peter propped his shotgun up against a rock and dumped his rucksack once again, quickly producing a small first aid kit from it.

Swiftly applying some antiseptic solution onto a gauze pad he then stuck it over the wounds on Stefan's neck, prompting a sharp hiss of pain from the sergeant.

"Ah, that stings!" He said with a wince. "Mate, you should get your mum to teach you the art of a gentle application for next time."

"Mum isn't in the army, and I'm not a nurse." Peter countered, resisting the urge to grin. "But I know that if she was here, sergeant. She would say that you'll live."

"Yeah, thanks to you."

"You've save my life often enough, Stefan. I'm just glad there was only the one of them and that it was just a juvenile."

"Yes, how did you know it was there?" The Doctor questioned. "You always seem to know when they're there, lieutenant?"

Peter scowled at the Time Lord.

"I listen out for them." He said, making it sound obvious. "If you weren't busy yapping away all of the time, you might actually hear them too."

"Perhaps I just haven't got as good a pair of ears as you?"

Peter noticed the accusation in his tone, and the Doctor could see it in his expression that he was debating on whether or not to challenge it. Eventually, however, he decided against it.

"I told you, I'm good at my job." He replied with a steady voice. "People might die otherwise."

"But that's three times now that you've known those creatures were there before anybody else?"

"I didn't realise that you were keeping count?"

"I'm just finding it a bit strange, that's all." The Doctor retorted. "How you're able to know when they're around, even from halfway across your camp?"

"Doctor, in my line of work you develop a gut feeling for when something dangerous is close to you." Peter reasoned with a shrug. "Turns out I happened to be right."

The Doctor was not convinced.

"But see, all of your men do the same job as you and none of them sensed that creature coming. Not even Davidson, not until it attacked him."

"Well, I guess there's a slight amount of luck involved and -"

"No, it's not luck with you lieutenant." The Doctor continued. "Because the first time, yeah I can agree that you might have gotten lucky. But that second one, the one down by the water…."

He stopped to think for a moment, replaying what he'd seen the solider do in the moments before he'd found the creature's body. "You sniffed it out, didn't you?"

He had watched as Peter had dipped his hand into the water, only to bring it back up to his nose. After that, Peter had known exactly where the dead creature was.

Peter looked surprised by this observation and offered no response.

"Doctor, he's just good at what he does." Stefan suddenly jumped in. "Besides, that thing stank even before it was dead. It's no wonder Peter caught a whiff of it."

"Oh, Stefan." The Time Lord sighed. "I know you know there's more to it than that."

"Doctor, what are you talking about?" Rose asked. "What's going on?"

"Yes, please do elaborate, Doctor?" Peter added sharply. "Because if you're accusing me of something, then come out and say it."

"All right." He stated, stepping forward. "I don't think that you've been entirely honest with us, lieutenant."

"Really? About what, might I ask?"

The Doctor took another step towards him.

Now he and Peter were no more than an arms length apart from one another. Taking a moment to stare into the younger man's bold grey eyes, the Doctor responded in kind with a look of suspicion.

"I'm not sure, yet." He told him. "But I am going to find out."

Peter let out an amused snort.

"Oh, so you don't actually know what you're talking about." He said, smugly. "Well, when you've figured it out please don't hesitate to let me know. I do like to learn new things."

"I'm sure you do." The Doctor replied. "But just so you're aware, Peter Argent. You may be able to tell when people are lying, but that doesn't stop anyone else from seeing it on your face too."

The confident smirk did not falter as the soldier eyed the Time Lord up and down with apparent curiosity.

"My god." He scoffed. "You're as mad as they say, aren't you?"

"Peter, whatever it is that you're hiding. I can help you, but only if -"

"Help? You think I need your help? The lieutenant interrupted. "Look, I don't know what paranoid, delusional theories you have running through your head, Doctor. But this is not the time nor the place."

"I couldn't disagree more, actually. You said it yourself, you need to be able trust those you're with in the field."

"By which I was referring to my team and those who have earned my trust." Peter countered. "You're nothing but a pain in the arse that I seem to have had no choice but to put up with."

"Charming." Rose muttered. "Do you treat everyone this nicely?"

Peter ignored her and continued to stare meanly at the Doctor.

"Maybe you should learn to trust other people?" The Time Lord ventured. "Let them in on whatever it is you and Stefan here are so very obviously covering up."

Surprisingly for the otherwise mild-mannered sergeant, his tone was one of anger as he spoke up.

"Tread carefully, Doctor." He warned, glowering at him. "You don't want this argument."

"Oh, I think I do, sergeant." He responded. "Because you know what's going on here, don't you?"

"If you mean that I watch Peter's back, then of course I do. He's my best friend for goodness sake!"

"You didn't seem to be all that concerned when he was attacked though, were you?" The Doctor challenged back. "But you were very quick to get him out of sight. Davidson is still laid up in bed, but the lieutenant here seems fighting fit to me."

"It was a scratch." Peter snapped. "I've had far worse doing this job."

"Yes you have, no doubt about it." The Doctor continued, pointing at his shoulder. "There is no way you escaped that fall with mere bruises. So, either you have a steel plate instead of a collarbone, or..."

He stopped himself, unable to come up with an alternative theory despite it being on the tip of his tongue.

Perhaps sensing that he had almost laid the argument to rest, Peter's smile returned.

"Or what, Doctor?" He asked. "Maybe I just didn't end up breaking my neck whilst saving your companion's life?"

"Doctor, maybe he's right?" Rose added. "He seems fine now, and wouldn't he be in a lot of pain if he had broken his arm or something?"

The Doctor considered this.

"Well they do say it is fleeting." He admitted. "Not to mention that time heals all wounds."

"Something which we are rapidly running out of." Peter told him, already turning away. "Even without you standing here and wasting it by -"

"But are you really going to carry on pretending?"

With a hand resting on his rucksack as he prepared to pick it up and carry on down the tunnel, Peter turned to face him with a huff of irritation.

"Pretending what?"

The Doctor now knew what had been on the tip of his tongue.

"It's strange that I didn't notice it straight away." He said. "But then again, you've had years of practice. You've learned how to come up with excuse after excuse when someone spots the very obvious truth about you."

Peter silence was enough to confirm that he was on the right track.

"Doctor, stop this now." Stefan ordered. "We haven't got time for this."

But the sudden defensiveness and lack of curiosity from the sergeant only confirmed his theory was right even more.

"You can't hide those eyes of yours, Peter." He continued. "Those eyes that have let you lead us all the way down here without a torch in your hand."

Peter was the only one of them who was not carrying one, and he had been up front the whole time when he might have needed it the most.

"What is your point, Doctor?" He asked. "If you have one at all, that is?"

"You know what I'm about to say."

It was a statement rather than a question, and he could see it in the lieutenant's face that he knew what he was about to say.

But a lifetime of covering his tracks and creatively calculating up ways to deceive people had the younger man lying about the truth almost automatically.

"I'm afraid I don't." Peter said. "So please, tell me?"

"All right, I'll just come out with it then."

"If it's not too much trouble?"

But at the last moment, the Doctor decided to give him one last chance to be honest. Rather than stating what was now so obvious, he chose instead to try and pull it out of Peter one last time.

"What you can do isn't normal, not around here at least." He stated. "But then again, normal is all about perspective."

Peter's confidence had wavered a little now, but he was still assured in himself enough to give nothing away.

"Doctor, nothing in my life has been normal." He said seriously. "I was dumped on a doorstep at three months old. I fight aliens for a living, and I am talking to a man who has two hearts and can travel in time."

"He's right, Doctor." Rose pointed out. "You're definitely not normal."

"To a human maybe." He replied. "Or another species I encounter. Isn't that so, Lieutenant Argent?"

He had looked Peter dead in the eye as he'd spoken, and there was no mistaking his meaning now.

"You did say it yourself, Doctor." He replied evenly. "I'm not like most other people. But that does not mean that I'm…"

He suddenly stopped himself, perhaps realising what he was about to say.

But the Doctor was not going to let him get away with it this time.

"That what, Peter?" He asked. "That you're not human?"

Peter wasn't shocked or angry, he wasn't confused or bewildered.

Fear flashed across his brilliant grey eyes and he suddenly looked like a small child who'd been caught out redhanded lying to his parents.

This time neither he or Stefan seemed able to muster a response.

"What did you just say?" Rose gasped. "Doctor?"

She was the only one to seem genuinely taken aback by the Doctor's revelation. Her head turned swiftly towards the lieutenant, staring at him as though he might suddenly sprout two heads or possibly even turn green like a martian.

"What's wrong, Peter?" The Doctor continued. "No clever comeback? Nothing you want to say for yourself now that I've rumbled your little secret?"

He was enjoying himself now the soldier's arrogance had been brought down a peg or two.

But then the Doctor saw anger replace the fear as a spark of light, one that he had previously thought he had imagined seeing, flashed across Peter's eyes. For the briefest of moments it gave off the impression that their colour had just changed.

Suddenly aware that the lieutenant was stood close enough to hit him, the Doctor warily eyed his left arm.

The strong bicep muscles twitched as Peter's fist clenched up into a tight ball, and he prepared himself for what might be a fairly painful blow. But quickly, he realised that the moment had passed.

Peter was not going to punch him. But that did not mean that he was finished saying his piece either.

"What the hell would you know about being human?" He said, his voice shaking slightly. "There is an innocent woman down here, possibly dying at the hands of these things, and you have the mordacity to..."

He suddenly turned away and forced himself to take in a deep breath.

Wary of making things worse, but equally wanting to know more, Rose hesitantly cleared her throat.

"We should carry on." She stated. "Peter's right, she needs our help."

But even though she was agreeing with him, Peter was clearly not willing to tolerate her or the Doctor any longer.

"Our help?" He spat out. "You think that you two are coming even a step further after…"

A soft, persistent beeping suddenly sprang up from his wrist.

"Thirty minutes." Stefan noted as his watch began to emitting the same noise. "Otherwise we won't make it back before dawn."

"Let's go." He agreed. "We have a job to do."

"Wait, you're just going to leave us here?" Rose asked. "Because if it's about -"

Peter had already turned away from her. But he stopped on his way briefly as his hand moved down towards his knife.

"Stay here if you want to, we'll escort you back but then you're both to leave and return to the TARDIS." He commanded as he handed her the weapon. "Just in case you run into another one."

His voice was no longer angry, but coldly unemotional as though he didn't know them at all.

Rose hesitantly took the knife from him and nodded.

But before Peter could withdraw his arm, however, the Doctor reached out and caught hold of his wrist.

What immediately stuck him was how warm Peter's skin was to the touch. It was like holding onto a hot water bottle, something which had nothing to do with the humid temperatures of the cave they were in either.

It was the hand which had been bitten too, the Doctor knew this because it was the one with the silver ring on his finger. Yet there was not a mark left on the skin to show for the damage the infant creature had done.

Choosing to ignore this for now, he softened his tone somewhat.

"Peter, I didn't mean..." He tried to say. "I didn't expect to -"

SMACK!

This time Peter did hit him.

Stumbling back into the rock behind him, the Doctor's head was spinning as he felt the sting of the slap across the lefthand side of his face.

"Did you just slap him?"

Rose found it strangely amusing that this strong, tall soldier had chosen to hit the Time Lord in such a way, even if it had left an angry red palm print on the Doctor's cheek.

Peter's angry snort was enough to make her think twice about laughing.

Flinching under his furious gaze, she watched as he picked up his rucksack and shotgun and marched off down the tunnel without further comment.

Stefan stayed where he was, scowling at the Doctor like an uncooperative bodyguard.

"Go. Now." He ordered. "Before he really loses his temper."

He then turned to leave too, but not before the Doctor made one last attempt to get some answers to his newfound discovery.

Risking a punch to the face once again, he grabbed the sergeant by the arm.

"Stefan, look I -"

"Whatever you're about to say, don't." The soldier hissed sharply, shaking him off. "Whatever you think you know, you don't. So leave it at that and for once, Doctor, do as you are told."

He then followed after Peter and vanished from sight.