The tunnel narrowed without warning.
Zigzagging at odd intervals, the path forward was only made all the more problematic by the large boulders and high walls that ran all the way down.
Peter and Stefan were immediately wary of progressing any further.
"I don't like this." The corporal muttered ahead. "Perfect place for an ambush."
Peter nodded in agreement.
"Eyes on. Stay alert."
"Copy that."
With the lieutenant leading the way down the tunnel, Rose was beginning to feel hemmed in as she and the Doctor remained sandwiched in between him and Stefan. There was room enough not to be forced into walking in single file, but the serious expressions on the faces of the two soldiers was enough to make her think twice about stepping out of line.
She shuddered at the thought of one of those creatures making an appearance down here.
Just as this idea crossed her mind, however, Peter came to a complete halt and held up a closed fist in the air as a signal for them all to stop too.
"Hang on." He whispered. "Something's not right."
Rose didn't dare ask what it was.
Twice now the soldier had proven capable of knowing something perilous was present before anyone else had. Sensing the danger through his friend, Stefan silently advanced past the Doctor and Rose as he brought up his shotgun and pointed in the direction Peter was looking in.
But there was nothing to aim at.
Peter hadn't lifted up his weapon, and there seemed to be no indication whatsoever that anything was even there at all.
Rose could no longer stand the tense silence.
"Is it another creature?" She whispered. "Or is it -"
"Shush!" Peter immediately hissed at her. "They hunt by sound, remember?"
They all stood and listened again. But there was nothing to hear aside from the odd droplet of water dripping down from the damp roof above, not to mention their shallow breaths.
"Are you sure there's one of them around?" The Doctor questioned. "You know, sound can echo off the walls and -"
Peter rounded on him, furious for the continued interruption.
"Yes, I am sure, Doctor." He snapped back. "Now shut up and let me -"
The single momentarily of lapsed concentration was exactly what the creature had been waiting for.
Its vantage point had been a gap in the rocks just above them that was no bigger than a football, and it was from there that the tiny clump of black skin and claws had pounced down from.
Stefan screamed as it landed on his chest.
"Ah!" He cried out. "Get it off!"
Still holding onto his shotgun, he accidentally squeezed the trigger.
BANG!
The boom of the Mossberg as it fired out the bullet sounded more like a cannon blast in such an enclosed space.
Had Peter been stood a few inches more to the left he would have ended up with a four-centimetre hole in his chest. Ducking instinctively as the barrel had swung his way, he covered his head with his arms as the wall next to him exploded in a shower of small rocks and dirt.
Almost as quickly, he was back on his feet and 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. Prying it off wasn't going to work and it might cause Stefan more harm should it dig itself into him any deeper.
Twirling his gun around like a baton, Peter struck the infant creature in the head with the butt of the weapon.
The alien was taken by surprise and let go, falling to the ground where it lay dazed by the heavy blow. Stefan immediately recoiled from it, collapsing against the rocky wall as he clutched his neck where he was bleeding from two large gashes.
Spinning his shotgun the right way around, Peter raised it and aimed the barrel straight at the creature as his finger wrapped itself around the trigger.
"Lieutenant, don't you dare!" The Doctor bellowed.
Peter looked up at him in surprise, his silver gaze meeting with the Time Lord's serious hazelnut one. It was followed by a glint of anger as, with a snort of annoyance, he lowered his weapon.
The infant creature had been knocked out. It was no threat to them for the moment and shooting it in such a state would have been a cold move, even if the alien had attacked them first.
With the soldier's impossibly bold eyes challenging him for a few more seconds, the Doctor watched as Peter's attention then swiftly shifted over to the stricken corporal.
"Stefan, you okay?" He asked. "How bad is it?"
Stefan's vest had taken most of the damage, but there were two deep cuts just to the left side of his neck. Wincing as he tentatively touched them with his fingertips, the corporal manoeuvred himself into a sitting position as Peter knelt down next to him.
"Ow, damn that hurts." He hissed. "It's just a scratch. Might need patching up though."
Until this point, Rose hadn't realised that she had been holding in her breath. Finally allowing herself to breath out, she eyed the two backpacks the soldiers were carrying.
"Do you have a first aid kit?" She asked. "I could -"
"Yes, of course we have one." Peter moodily snapped at her. "Why wouldn't we?"
Rose didn't feel the need to argue back this time. They had all been on their feet for hours now and it was hot down here. Topped with the constant attacks from the aliens and Stefan getting hurt….
No sense fanning the flames, she thought.
Setting aside his shotgun and slipping off his rucksack, Peter quickly pulled out a small white box. From it, he removed a bottle of antiseptic solution and a large square gauze pad.
Stefan pulled away the collar of his shirt in preparation.
"Remember, just press it on gently." He coached. "You don't need to strangle the bloody thing."
Peter smirked back as he poured the solution onto the soft white pad.
"Calm down matron. I've got this." He chuckled. "But it is going to sting."
Stefan bit back a yelp of pain as the covering was stuck over his wounds.
"Ouch!" He complained. "I said gently, Peter. You need to 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. "Now don't be such a baby and get your feet."
Standing up, Peter held out a hand towards the corporal.
"Just because you've taken on one of the big ones," Stefan replied, smiling as he took it.
"Two of them, actually." Peter countered. "Plus this one we can call a… Ah!"
The lieutenant fell against the wall quite suddenly as he clamped his hands over his ears. Grimacing as though in tremendous pain, he had become unable to maintain his balance as a result.
Stefan was immediately at his side, holding onto him for support.
But the strangest thing was that there was no sound to warrant such a reaction, nothing that any of them could hear that would cause such discomfort. Looking around, however, Stefan knew there had to be something. His friend wouldn't have just begun to act this way for no reason. There had to be…
Turning around, he saw the Doctor. In the Time Lord's hand was the sonic screwdriver.
He was pointing at the unconscious creature, the small tip with flickering with a blue light despite the lacking of an audible signal.
"Stop that!" Stefan ordered. "Stop whatever it is you're doing."
The Doctor obliged at once and lowered the small metal device.
"Just making sure this not so innocent infant doesn't trouble us for a few hours." He explained. "Problem?"
For the first time, Stefan looked genuinely angry.
"Yes, there damn well -"
"Stefan." Peter quickly intervened. "Leave it."
Though still frustrated, his friend did not say anything more and turned away from the Doctor. Peter had recovered himself now and had stood himself up straight again. It was as though the whole thing had never occurred at all.
Rose couldn't understand any of it.
"What was that all about?" She asked.
"I'm not sure." The Doctor said casually. "Sensitive hearing, lieutenant?"
Neither Peter nor Stefan responded to this.
"But there wasn't any sound?" Rose pointed out when they failed to speak. "What happened?"
"Oh, there was a sound, Rose." The Doctor continued. "Sound is around us all the time. Just depends on whether or not your ears are tuned in enough to pick it up."
Still holding onto the sonic screwdriver, he began gently flipping it up into the air before catching it again. "For example, that sound you didn't hear just then was around sixty-five kilohertz in terms of frequency." He stated. "Impossible to hear for some. Painfully loud for others."
There was accusation in his voice now as he stared straight at Peter.
The lieutenant glared back but it was only for a moment. Then he backed down and looked away, his eyes travelling to the floor as though he already knew what was about to come.
Unsurprisingly, it was Stefan who came to his rescue.
"Doctor, we haven't got time for this." He told him. "Just put that thing away and come on."
The Doctor let out a long sigh of disappointment as he pocketed the sonic screwdriver.
"Oh, Stefan." He began. "I'd be let down by the lack of curiosity, expect I know you already know what's going on here. Don't you?"
"What is going on?" Rose asked. "What are they not telling us?"
She was beginning to feel very left out in the dark. It was an ironic conclusion given their current surroundings. Fortunately, the Doctor seemed keen enough to explain whilst the two soldiers were for now not doing anything to stop him.
"Yes, they have been rather secretive with us, haven't they?" He questioned. "Here's me thinking it was just down to you two being best friends. But then I thought, Rose, it was quite strange how on three separate occasions now the lieutenant here has to know that these pesky aliens were nearby."
Peter was scowling at the Time Lord again.
"I listen out for them." He said, making it sound completely obvious. "If you weren't busy yapping away all of the time, you might hear them too."
"Yeah, but my ears can only reach twenty or so kilohertz. Nowhere near as good as yours." The Doctor countered. "But you didn't just hear them coming, did you?"
Taking Rose's torch from her, he quickly held it up and pointed it straight at Peter's face. The soldier winced and immediately looked away, clenching his eyes shut against the harsh light.
"What the hell does that prove?" Stefan protested. "Doctor, this -"
"Not once have I seen you use a torch tonight, Peter. This torch I'm holding is yours, but you just handed it over to Rose without a second thought." The Time Lord carried on. "But I'm going to guess that's because the tapetum lucidum in your eyes makes it a bit easier to see down here."
"Translation, Doctor?" Rose asked. "What is that?"
"Sorry, yes." He replied. "It's a special light-reflecting surface right behind the retinas of the eyes. Helps you see in the dark, no carrots needed at all."
Though this was supposed to help her understand, it only made the whole situation more confusing. But as Rose grew more and more puzzled, Stefan was becoming just as increasingly irate.
"Will you just stop talking and -"
"No, I don't think I will." The Doctor retaliated. "Not until I've gone over everything. You could say I'm checking for any mistakes, Stefan. Wouldn't want that to happen again, would I?"
Peter was remaining stoically silent.
The tension in his strong jaw, however, showed a wave of brewing anger that might implode at any moment. He had been caught out lying about something big, and it was something that he didn't want becoming public knowledge.
Regardless, the Doctor continued telling them all about it. "But then again maybe we should start from the beginning? Your coat, lieutenant." He pointed out. "Now why didn't you take it out with you, or your gun for that matter? It's February and killer bats are running around, both of which you are acutely aware of."
"He said he didn't need either." Rose suddenly remembered. "Also, at the pool, you weren't even shivering. How come?"
"Oh! Very good, Rose." The Doctor cheered. "Still, perhaps Peter is right. Maybe he doesn't need either. Maybe he had another form of defence against the cold and against the monsters that no one else has. One that no one else knows about."
His expression then hardened as he took a step forward.
Stefan moved to step in between the Doctor and Peter as he approached. But his friend held out an arm to stop him and watched as the Time Lord came over, stopping only when they were a few inches apart.
"Not many people know what a metamorphic race is off of the top of their head." The Doctor then stated. "Not many people can walk away from a twenty-meter drop, headfirst I might add. Nor can they take a metal bar to the head without even so much as a bruise."
He paused, contemplating his next words. "Tell me, Peter. You're cleverer than most, I think." He continued. "What happens if you have a protective system of regenerative cells?"
Peter didn't answer him.
"Regenerative?" Rose repeated. "Like when you changed, Doctor?"
"No, not quite." He mused. "See I'd have to be dying for it to kick in. The lieutenant here can -"
"Enough!"
Peter bellowed out the single word.
But the Doctor wasn't intimidated, not when he was on such a roll. He liked discovering something new and it had become fun picking apart the clues as to who Peter Argent was.
"Then maybe you should stop pretending, Peter." He told him. "Stop lying to us and admit that you're not -"
But that was as far as he got.
Peter moved so quickly that the next thing the Doctor knew he was being pinned against the wall as the cold metal of the lieutenant's handgun barrel was pressed against the side of his head.
"Peter, no!" Stefan shouted. "Stand down right now!"
"Get off him!" Rose cried out.
But neither of them went any further as the Doctor shushed them and motioned with a wave of his arm for them both to stay calm.
"It's okay." He reassured. "We're all okay here."
To anyone else, the situation was far from okay. Peter still had the gun pointed at this head and he hadn't hesitated in using it before.
But the Doctor knew he wouldn't this time for one very good reason.
The lieutenant wasn't shocked or angry, he wasn't confused or bewildered. It was fear which had embedded itself into his brilliant grey eyes as a spark of previously light flashed across them. The Time Lord had thought that he had imagined it the last time. But now he was sure that the silver in his eyes did not always stay the same colour.
Peter flinched as a hand rested on his shoulder.
"Mate, step down," Stefan told him gently. "Lower the gun. He's not worth it."
Looking quite pale now as he seemed to realised what he had nearly just done, Peter obliged and stumbled away. He handed Stefan his gun as he did so, obviously not trusting himself to hold onto it for the moment.
Watching as the soldier turned his back on them, receiving a few whispered words of reassurance from Stefan, Rose felt a pang of sympathy for him amongst the rush of relief she had for him having not just shot the Doctor in the head.
Desperation and fear. That's all it was.
She had a very good idea now of what it was Peter was hiding from them, what he was prepared to kill for it not to be said out loud.
But where did it leave them now?
There was still at least one more alien to encounter along with the missing woman to find. They were running out of time if she was going to be rescued alive and well.
"We need to carry on." She found herself saying. "We need to find this woman and stop these things."
She was pleased to see the Doctor smile at her. Stepping away from the wall he took her hand in his and nodded in agreement.
"Yes." He said. "We need to do exactly that. Don't we?"
With Peter still turned away from them, only Stefan responded as the two time travellers looked their way.
"What the hell is wrong with you?" He spat at the Doctor. "Are you just completely incapable of…"
He stopped himself, too aghast to come up with anything else to say. Instead, he turned to Peter and laid a comforting hand on his shoulder.
This small act was enough to make to Doctor realise how much damage his investigation had done.
"I'm sorry." He said. "Peter, I'm sorry. If you don't want to tell us… If you don't want to say… That's fine."
Rose found herself holding her breath again as they all waited to see how Peter would respond to this. Not moving for a moment, the lieutenant continued to stare down the tunnel ahead of them. When he did eventually speak, it was in such a whisper that the Doctor thought at first that he misheard him.
"You need to go, Doctor."
"I'm sorry?"
"I said that you need to leave," Peter repeated, turning around to face him and Rose now. "Go back to the TARDIS and leave me and my mission alone."
Without waiting for an answer, the lieutenant picked up his rucksack and retrieved his shotgun. Holding out a hand towards Stefan, the corporal hesitated only for a moment as he handed him back his pistol.
"Wait, you're just going to leave us here?" Rose realised. "Because if it's about -"
The small high pitched screech coming from the floor right behind her made Rose freeze in terror. Knowing she shouldn't but equally unable to resist, she turned around and looked down.
The infant alien had woken up and was staring back up at her, its whole body held tense like a coil desperate to spring back up. The tiny claws were turfing up the soil in preparation…
Rose screamed as it leapt for her.
She didn't even hear the bullet which killed it. But she did see the tiny creature explode mid-air and be reduced to a pile of burning matter in a split second as the shotgun blast cut through its small body.
With the barrel of his Mossberg still smoking, Peter lowered his weapon.
"You okay?" He enquired. "Did it -"
"N-no, no I'm fine."
Rose slowly managed to turn around and face him and was astounded once again how calm he was. She could have very well just tripped over a stray rock given the unflinching way he had just dispatched with the creature that was now splattered in pieces all over the ground.
"So much for you keeping it off our backs," Stefan remarked, eyeing the Doctor. "Or were you just making a point?"
It was the Doctor's turn to remain silent.
Peter was already turning to leave, his decision unchanged about them splitting up as he marched off down the tunnel without further comment.
Stefan remained behind only long enough to unclip his knife and hand it over towards Rose.
"What's this for?" She asked. "I don't -"
"It's just in case." He told her. "But you two need to go, now. Before he really loses his temper."
Risking the corporal's own wrath, the Doctor reached out and caught hold of the soldier by the arm.
"Stefan, look I -"
"Whatever you're about to say, don't." The soldier hissed, 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.