The moment I land back on Earth, I can feel it: something is different. I stumble out of the teleportation chamber and allow myself to catch my breath. I know of the Sontarans, I know how deadly they are. I should have been more scared than that. Whatever power the Coin has, it's affected me in some way. It must have.

My glance passes over the giant windows of Luke's personal quarters. Something catches my eye. London should be visible from the hill Rattigan Academy is built upon but I can barely see it. The entire city has been covered in a thick fog. Only it isn't fog. It must be a gas of some sort, I can smell it even from here. Sharp and bitter. ATMOS was a threat after all, it must have released this poison. For what, I don't know.

I am stranded here. There were no cars in the academy driveway — none that aren't ATMOS, that is — and I'm not even certain as to how long it will take to return to UNIT. My phone is gone and my chances of contacting the Doctor limited to none.

The chamber emits a low hum, pulsing with purple light. Luke appears.

In an instant, I have him pinned to the wall by the front of his t-shirt. "What did you do?"

He immediately cowers, squeaking with fear, "It's not my fault! They wanted me to!"

I don't have the patience for this. Keeping a hold on him, I march him over to the window and roughly shoving his face against the glass. "Look at that," I yell. "What, do you still think this is some game? People are going to die!"

I feel his shoulders tense under my hands, a sob escaping him. My grip loosens. But then I catch a glimpse of his reflection. It wasn't crying I heard. In the face of this horror, he is laughing. "I know, right? Isn't that so cool? I mean, I created that!"

My stomach turns. In my life, I have seen some evil things, but sometimes I still find myself surprised by the atrocities that people commit. Sickened to even touch him, I pace the length of the room and hug myself tightly. My anger is strong enough to bring me to a stop soon after, scowling at him as he continues to watch the fog thicken. "You may be smarter than the average human, but you are not better than them. You don't get to choose when they die. The Gods will punish you for this. You will burn."

"'The Gods'?" he scoffs. "You seriously believe in all that? I thought you were smart."

"Smarter than you, smart enough to know when someone is using me. And what do you believe in, Luke? Science? The Sontarans? Because believe me when I say that they are not gods, they're just bloodthirsty idiots... and you're just as bad for helping them."

With nothing else to say, no argument to give, he storms out of the room. I follow after him, ready for the next provocation to get my rage out. We pass through the double doors and onto the front steps. In the garden below, the students watch in horror as London sinks further below the murky clouds. "Leave it. Turn away," he says. "Civilisation is falling."

One of the eldest steps forwards, his face sickly pale and drained of colour. "But it's all over the news, sir, it's everywhere — Paris and New York."

I look on in disgust as he chuckles to himself, making his way down to them. "It's time I told you, all of you. It's time I revealed what our work has been for. Come." Reluctantly, they follow him inside and I tag along, curious to see exactly how he expects to explain this away. He stops in the middle of the lab and gestures to the projects set out along the workbenches. "All this stuff we've been building, all this... invention, where's it been heading? Gravity densifiers, hydroponics, atmospheric conversion, ecoshells. More than enough to build a brand new world!"

"Luke, we haven't got time for this. I've got to find my parents."

The boy's worry appears to amuse him. Rolling his eyes, he continues, "Oh, but this isn't just theory. I'm talking Planetfall for all of us. A brand new start for a brand new human race. Look." Grabbing a remote, he starts up a hologram projection from the overhead lamp. Two tiny spheres appear, one orbiting the other, surrounded by several orbital rings. "A new world... far out, beyond Alpha Geminorum, just waiting for us. Its official designation is Castor Thirty-Six. I think of it as Earth 0.2. I did wonder about Rattigan's World," he arrogantly adds, "but we can take a vote on that."

I offer no help for the students to understand. I don't need to. Luke is doing perfectly well in turning them against him, that much is perfectly clear. "What the hell are you on about?" the boy huffs.

"This is where we're going. I have partners willing to take us there."

Scoffing, one of the girls walks right through the projection on her way to the door. I don't bother to suppress my smirk. "He's lost it. I haven't got time for this."

"Excuse me," he snaps. "I haven't finished. Where are you going?"

"To find my brother."

"I didn't say you could leave."

The walls are coming down. It's clear on each and every face that this is not what they signed up for. "I told him to put ATMOS in his car, I've got to go and help him!"

He pulls a gun from his waistband. "Stay where you are!"

Like the others, I freeze. It should have been so easy to spot but I had no clue. She stumbles back. "What do you think you're doing?"

"Luke, put that down."

He turns on the boy. "I did this for you. Don't you see?" I try to creep up behind him but he turns again, the gun just inches from my chest. His eyes are even wilder than before, I'm starting to wonder whether or not he'll pull the trigger. "We've spent all our lives excluded. The clever ones. Th-They laughed at us and pulled us down, those ordinary people out there. Those... cattle. This is our chance to leave them behind."

"You mean, you want them dead?"

"And I chose you to survive! With Planetfall, we can start again. We can build and breed, we can prosper, we can do anything."

The girl hesitates, raising her eyebrows in disbelief. "We're going to 'breed'?"

"I've designed a mating programme. I've planned the whole thing."

I wrinkle my nose. She laughs, "Well, then. Shoot me."

"Stay where you are." He raises the weapon again but she notices the tremor in his aim. With another rolling of her eyes, she leaves. "Stay where you are, I said!" A few of the other students join her. "Stay where you are. That's an order!"

In no time, only the boy remains. "'Castor Thirty-Six'? You're sick."

Now it's just us. His hands fall back to his sides in defeat. Still, he calls after them, "Guess that just proves it. I'm cleverer than you. I'm cleverer than everyone! D'you hear me? I'm clever!"

Arching an eyebrow as he stamps his foot, I approach. "Of course you are. That must be why you act like a three-year-old. Oh, for Minerva's sake, give me that!" Luke makes no attempt to resist as I snatch the gun out of his hands. I look it over with disinterest. "You left the safety on. Honestly, of all of the humans to pick for their little stunt, the Sontarans went for a little boy. It's sad."

I hear a quiet sniffle. Hurriedly wiping his eyes, he stands up straight and tries his best to put on a brave face. "What are you gonna do with me, then? Kill me?"

"Gods, no. I don't kill unless I have to. Even then, I don't harm kids." With one more step towards him, I tower over him as I slip the gun into the waistband of my trousers. "Now, you're going to help me fix this. Or do I need to call your parents?"

Before I have a chance to realise his plan, he darts away. I sprint after him, all the way back to his room. My hand reaches to grab onto him just as he steps into the teleportation chamber. Cursing loudly, I kick the side of it.

There is no use following after him. This time, the Sontarans know better. They will shoot me down immediately. I head back to the lab and start to sort through the various bits of machinery and equipment.

I can't stop the Sontarans on my own — not in any way that adheres to my morals. Whatever ATMOS does can't be undone now, but maybe I can remove this gas.

Taking a deep breath, I try to clear my head. My hair is secured in a low bun and my clothes are protected by a lab coat, goggles and gloves. An ATMOS prototype sits at the edge of one of the workbenches. I bring it in front of me and loosen the screws to open up the external casing of the flat device. It breaks off into halves and I start my investigation.

It isn't long before I hear the hum of the teleport chamber again. This time I let Luke come to me. It takes a few minutes before the distant noise of his sobbing quietens and footsteps reach the doorway of the lab. Not looking up from the dismantled ATMOS converter, I wearily ask, "What do your masters say?"

He sniffs, wiping his face again. His eyes are reddened from crying, his voice coming out strangled. "They lied to me. Planetfall, it wasn't real. They were gonna shoot the students as soon as they came onboard the warship."

As irritated as I am, I can't help but pity him. Sighing, I down my tools. "All right, jokes aside, do you actually want me to call your parents?"

The boy shakes his head quietly. "No use."

"Ah, I get it. Orphan?" He shrugs in response. "Yeah, there's a surprise. Are you done running away now?"

"I wasn't running away."

There's something about his tone, the stubbornness of it, that amuses me. I glance over my shoulder at him with a knowing smile. "Weren't you?"

It's enough to weaken his argument. "Well," he says with another shrug, "I did before, I guess."

"It's all you've been doing, isn't it? Come on, Luke, you're not stupid. All of this — the school, the Sontarans, Planetfall — you're doing it to escape. You don't think anyone understands you."

Still scowling, he slumps down on the stool beside me and mutters, "They don't."

"There's always someone who can understand. Sometimes, it takes a bit of looking, a bit of opening up, but I can guarantee you there isn't a single human on this planet who has gone through a completely unique experience. You're still young, you'll figure it out." I pass him a pair of goggles. "But vanishing off to some distant planet is not the way to do it. There's still time to fix things if you're open to it."

He folds his arms, pouting sulkily.

I let out a sigh and take a glass cylinder from the bench, fixing it onto the top of my new machine-in-the-making. "Probably hard to believe but I was seventeen once. I thought I was clever too — maybe I was, who's to say? — but it took the loss of my planet to make me see the bigger picture. See, my home is dying right now... because of pollution, poison in the air. Funny how things come full circle. So take it from someone who knows: you do not want the death of your planet to be your wake-up call. No kid should know what it's like to take a life. Although you already have... but let's get ahead before any more join that list, yeah? You'll thank me later."

Another moment passes in silence. Finally, he clears his throat. "What are you making?"

"Something to get rid of that gas before Earth chokes to death. Pass me those tweezers, will you?" I wait, my hand open and waiting. He hands them over.

—

An hour later, the teleport chamber buzzes to life once more. We both jump up and race through to Luke's rooms. The second he sees the Doctor, Donna and Martha, he snatches the gun from my waistband and aims it at them with renewed panic. "Don't tell anyone what I did. It wasn't my fault, the Sontarans lied to me!"

On his way past, the Doctor snatches the weapon away and throws it aside. "If  I see one more gun..." He pauses in front of me, his gaze travelling over me, from my hair still clumped from my swim in the river to my dirtied clothes. "Are you all right?"

"Yeah," I mumble half-heartedly, heading over to Martha instead. Her clothes have been replaced by a white hospital gown and his trench coat, its relative size swamping her. "I'm assuming you know we've got a clone on the loose."

"It's fine," she sighs. "She's gone."

I nod awkwardly. "Well, she gave me a good turn-around. Thought you were in danger, Doctor. Sped up here, got thrown in the river."

Donna wrinkles her nose as she approaches us. "Explains the smell. You know, that coat sort of works."

"Feel like a kid in my dad's clothes," she sheepishly replies.

Leading them through to the lab, I direct the Doctor to my clumsily put-together contraption. "Been working on something to get rid of the gas. Reckon it should help clear the air up. If I'm right and it's caesofine gas, we won't last much longer before we all choke to death. Any clue as to why the Sontarans are going all evil-genius with this one?"

He picks the contraption up and looks it over, sparing me an impressed glance. "This is good. This is very good."

"Thanks."

"As for the gas, I'm assuming that's why the Sontarans stopped the missiles, to hold back—"

"I'm sorry," I scoff, "'the missiles'? I'm gone for a couple hours and things finally get high-stakes?"

Ignoring my comment, he grabs a hammer and gives the device a good whack to knock it into shape. "'Cause caesofine gas is volatile, so they used you to stop the nuclear attack, Martha. Ground-to-air engagement could've sparked it off."

She frowns. "What, like, set fire to the atmosphere?"

"Yeah. They need the gas to breed their army. And all the time we had Luke here — the dream factory. Planning a little trip, were we?"

I spare the boy a look over my shoulder as I rush to assemble the rest of the device, the Doctor and I so used to working together that we never collide, weaving around each other and silently communicating the next piece of this puzzle. Luke fixes his eyes on the ground and quietly responds, "They promised me a new world."

"And you built equipment, ready to terraform El Mondo Luko, so that humans could live there and breathe the air with this!" We both slide on the last component, a metal casing. The second it connects, red light shines through the slots marking its surface. He threads the connected cabling around his neck and lifts up the tall, cylindrical machine. "An atmospheric convertor."

We race after him onto the lawn at the front of the academy, setting the machine down. Donna pauses as she sees the blur of fog in the distance. "That's London. You can't even see it. My family's in there."

Oblivious, he keeps his ear to the device as I turn the small dial on its side. "If we can get this on the right setting..."

"Hold on. You said the atmosphere would ignite."

"Yeah. I did, didn't I? Inara, would you do us the honour?" he says, offering the button to me.

I press it, causing a flare of light to shoot up from the device and into the clouds above. The spark brightens their greyness. The fire spreads, shooting out in all directions like a tidal wave.

When the flames dissipate, the sky is blue once more.

Before I can react, Luke throws his arms around me. "You're a genius!"

"Thanks, kid."

Our celebrations are cut short, though, as the Doctor grabs it and makes for the doors once more. "Now we're in trouble."

"What? What is it?"

He doesn't stop until he is back in the teleportation chamber. "Right. So. Donna. Thank you, for everything. Martha, you too — oh... so many times. Luke, do something clever with your life. And Inara..."

I frown, watching in bewildered silence as his eyes meet mine. A part of me knows what he is doing, the other doesn't want to think about it. "What?" I prompt.

He gulps, looking back down at the flashing lights of the atmospheric convertor. "Well, um..."

"You're saying goodbye," Donna realises in a horrified whisper.

"Sontarans are never defeated. They'll be getting ready for war. And, well, you know, I've recalibrated this for Sontaran air, so..."

He won't look at me now. I'm starting to realise why but say nothing. It is Martha who speaks up next, horribly quiet, "You're gonna ignite them."

"You'll kill yourself."

He doesn't respond to Donna. Martha huffs, desperate for a solution. "Just send that thing up on its own. I don't know, put it on a delay."

"I can't."

"Why not?"

"I've got to give them a choice," he replies. I already know what their choice will be, the Sontarans will never surrender. I also know that he'll only bring himself to do such a thing if he knows that it is the only option.

Against his better judgement, he looks to me again, hoping for some kind of goodbye. When I give him nothing, he reaches for the button on the chamber wall. I step into it just as the lights start to brighten.

Now in the darkness of the Sontaran ship's bridge, the Doctor realises. He turns on me in a sudden panic. "What are you doing?"

I snatch the device from him and set it down, holding its button at the ready. "I made this damn thing. Did you really think I'd let you use it without me?"

"Oh, excellent." Freezing, we find a horde of Sontarans all watching us.

He takes the button back from me. "General Staal, you know what this is but there's one more option. You can go, just leave. Sontaran High Command need never know what happened here."

"Your stratagem would be wise if Sontarans feared death. But we do not," he barks. "At arms!"

I don't miss the way that he steps forwards a little just as they take out their guns, putting himself in front of me. "I'll do it, Staal. If it saves the Earth, I'll do it. Get out of here, Inara."

"A warrior doesn't talk, he acts!"

"I am giving you the chance to leave."

It is becoming increasingly apparent that he is pleading with them, not warning. The General scoffs, "And miss the glory of this moment?"

The speakers overhead buzz, "All weapons targeting Earth, sir. Firing in twenty."

"I'm warning you!"

"And I salute you! Take aim!"

"Shoot me, I'm still gonna press this."

Growing impatient, I try to wrestle the button away from him. "Don't be stupid, Doctor. Either do it or give it to me."

He struggles but can't outmatch my strength for long. "Inara, I'm serious, go."

"Not a chance!"

"Firing in fifteen."

I manage to tear it from his grip and push him to the back of the chamber. Staal roars, "For the glory of Sontar! Sontar-ha! Sontar-ha!"

As the cry picks up, I fight to have my voice heard. My hand poises but I can't bring myself to do it just yet. Only ten seconds left now. "Doctor, get out right now!"

"I won't leave you!"

I get ready to activate the device. Just as my finger comes down, light surrounds me again. My body hits the floor of the chamber, colliding with the Doctor, only to find that we are back in the academy again.

Martha rushes to our side and I return her relieved hug, aware of a hand at my back. The Doctor and I don't utter a word to one another, nor meet each other's gaze. Seeing Donna approach, we both look up. She slaps him in the arm but thinks better of it, hugging the both of us tightly.

"Where's Luke?" I shakily ask.

"He said he was doing something clever." Nothing else is said but it isn't hard to understand — he took our place. He was told to do something clever with his life so he saved ours. It cost him his.

——————

Shaken, we return to the Tardis. I bring Martha to the wardrobe in search of some fresh clothes and pick out some for myself after a long shower, hiding behind the changing screen. I can hear her footsteps as she paces along the aisles between jumbled racks of clothes. "So," I