Fixed Point n. An event in space-time that cannot be altered.

"You have to think of it like a sandwich," he says, to her confusion.

"Like bread can't be made back to dough?"

"What?" Now he is scowling in misunderstanding. "What are you talking about?"

"A fixed-point," she reiterates. "What are they, and why can't we change them?"

They are sitting on deckchairs dragged out of a TARDIS storeroom; heirloom pieces stamped Property of Blackpool Pier. The beach here is a scrubby, industrial sort; scum bubbles and floating rainbows of spilled oil bobbing on the waves. Across the bay, the enormous drill-site is half-shrouded in the early morning mist.

"Oh, yes, think of a sandwich," he repeats, unhelpfully.

"Any particular kind?"

"That's the point. You go into a shop, you buy a sandwich. You make a choice. Chicken or egg mayonnaise."

"And?"

"And every time you make a choice, any of us makes a choice, somewhere out there a different version of you takes the other option."

"I've heard that theory before. So, there's hundreds of parallel universes out there where I made different decisions?"

"Yes and no. Back to the sandwich. Which do you choose?"

"Oh. Um, egg mayo."

An opportunistic seagull lands near her foot, appraising her hopefully. A few seconds later the rumble of the drill causes it to take flight in alarm. "Don't worry," he says, as the armrests of her chair start to vibrate with the working of the nearby bit. "We've got a while yet. You picked egg mayo. You get salmonella, sorry."

"Okay then, I pick chicken."

"Nope, you still get food poisoning."

She laughs. "What are you trying to tell me? That some choices are an illusion?"

"Not an illusion exactly, it's just they don't matter very much. Your timeline diverges for a while but snaps back together again. You remember the sandwich made you ill; but was it the chicken or the egg? You can't recall. Because it was both."

She ponders this for a while, watching a plastic bottle ride the surf; trying to remember a situation like the one he has just described. "So that's a fixed point? But what about if I get a salad instead? Or shop somewhere else?"

"Precisely; that example, there's a way around the choice. But sometimes you get a…a stacking up of all of those meaningless choices on top of one another. A critical convergence of timelines that makes one particular outcome inevitable. They're rare. But they do happen."

"And you can… feel them?"

He nods, turning his face into the breeze that is beginning to pick up. "Like an oncoming storm."

He looks terribly dramatic. She can't help but laugh, puncturing the moment somewhat. "What must happen here then?"

"Wait and see."

She unwraps her sandwiches with a sigh. "These better not give me food poisoning."

She has barely finished her crusts when the explosion happens. A rose of fire, blooming out of the roof of the largest building; a booming crack of immense sound.

"Some kind of accident?" she says, trying to make sense of what she is seeing. Even across the bay she can hear the wail of the sirens. "Doctor?" She turns to find him rigid in his deckchair, white knuckled fingers digging into the armrests; a man resisting torturous agony. "Doctor! What's wrong?"

"Someone," he says, through gritted teeth, "Is trying to change the fixed-point."

"But they can't, can they? I thought that was the point."

He shakes his head, breathing shallow through the pain. "No, they can't. But the bits around the-ah!" He gasps. "The bits around the edges are mutable."

"Bits?"

He knows that stern set to her jaw too well. "People," he admits. The crackling agony in his bones is fading to a dull ache as the multiverse settles into a new configuration.

"Tell me right now," she says, furious, "Exactly what we came here to see."

"The drill disturbs creatures that have been sleeping under the earth for centuries. They come out, and for once humanity does the right thing. You bend and change; give them their peace and space. Drill somewhere else. This beach becomes part of a nature reserve. I was going to do the before and after."

Her anger subsides a little. "Then the explosion…?"

He shakes his head. "Never happened."

"Can you walk?"

"I'll be fine," he snaps.

"Then come on." She holds out her hand to help him up. "Let's go do what we do best. Let's go save people."

Sophie's Choice. phr. 1. The choice between two unbearable options.

2. "Sometimes the only choices you have are bad ones.

But you still have to choose."

And here they are again, on the knife-edge between good and greater good. "Clara," he says, "I have to free them. If they remain trapped here today-"

"What?" she snaps "Someone will have proved you wrong and changed a fixed point?"

Smoke billows up through the grated stairwell, the heat intense. "They're not my rules!" he shouts. "The Universe will find some other way! The Flemeth must be freed. If I don't do this now, this whole planet is going to crack like an egg. Millions will die. I need the TARDIS."

"Then we'll make two trips! It's a bloody time machine!" She coughs, choking on a lungful of the black smoke.

"We can't. Every trip we make in time is a wound, Clara. I promise I will explain everything to you." He flinches as another industrial unit along the lot erupts in a tongue of flame. "But right now you have to trust me. One trip. That's all this point can take. We have to free the Flemeth."

"No," she says, shaking her head. "I can't leave them here."

She is serious, he knows; an immovable object to his unstoppable force. He hates her for this stubbornness; her refusal to accept that sometimes, just sometimes, he really does know what's best. And he loves her for it. He loves her so very much for refusing to bend to his will, even in the face of all his lordly righteousness. Clara Oswald will not let these people die. Even if it kills her. She is not his servant; not some willing assistant, but his companion. His friend. And this is the price.

"Okay," he says heavily, tossing her the sonic screwdriver. "Setting four seven two should help with the heavy lifting. We passed a short-range scow on the way in that still looked drivable. You remember how to pilot a Coriolanus class ship?"

"Yes."

"Good, it shouldn't be too different."

"What about you?"

"I'm going to try and stop a planet collapsing. Apparently without my screwdriver."

There is no time for a lengthy goodbye. She has minutes to free the children, trapped in the shattered remains of their classroom, before the flames consume them all. Her future is invisible to him, shrouded in the grey of the changing timeline all around them. He is all too aware that this could be the very last time he sees her.

"Good luck," she says, voice tight.

"Yeah, you too," he manages. Somehow, it doesn't seem quite enough. As she starts to turn away, he grabs her free hand, pulling her towards him roughly. He kisses her clumsily on the cheek as she hugs him fiercely for a brief second. "See you on the other side."

He does not stay to watch her run back to those she would risk her life to save. He turns on his heel instead; to the fiery abyss that awaits.

The TARDIS rises from the smoking crater, spinning like a top, flanked by two grateful Flemeth. They are dragon-like; crocodile jawed and leathery winged: a true wonder of the cosmos. They fly onwards and up, towards the blue sky, as he pilots the TARDIS across the bay to where a short-range hoverscow is in serious trouble. One of her engines has been clipped by explosive debris, firing intermittently, and trailing blueish smoke. As he watches it finally stalls; the craft flips like a car clipping kerbside, impacting the water with a tremendous splash.

"No, no, no, no, no!" He isn't aware that he is shouting, as he throws the TARDIS after her, dropping into the water like a cannon ball.

He opens the TARDIS doors to see the sinking craft heading for the sea bed. The emergency hatch blows outward; a figure in the doorway struggling to resist the rushing tide of water that flows into the stricken ship. It is Clara, bundling life-jacketed children out to safety. They bob towards the surface like the pieces of plastic they watched on the beach; what feels like a lifetime ago.

He extends the TARDIS airshell, allowing her to surface, and steps out onto the waves using a cushion of atmosphere. "Gotcha!" he says, grabbing hold of the first piece of flotsam by his lifejacket; a little boy perhaps six or so; gasping and choking on swallowed water. He hauls him out of the sea and onto the airshell platform, patting him roughly on the back as he retches. Others surface all around, crying and screaming, but at least that means they're alive and breathing.

She rises last; a few feet to his right. Catching sight of the TARDIS she grins and begins swimming doggedly over. Rescue craft from the nearby coastguard are pulling up around them, casting out lines to the schoolchildren in the water.

He pulls her from the surf; cold and wet; caring not a bit that his suit is soaked as he embraces her. She buries her face in his neck for a moment.

"Did we win?" she asks, when she eventually lifts her face.

"Yeah," he says, still gripping her arms, "Yeah, we did."

"Oh, good," she says, pulling him back in close.

"It was a rogue Time Agent," he says.

She is lounging in his armchair, previously engrossed in a novel, which she folds shut at his words. "What was?"

"The explosion. He lost his sister in the original time-line; she was part of the drill team that fell through into their nest."

"Did he save her?"

He gives her a sharp look. "Fixed-point."

"Her death was?"

He flicks a few buttons and switches on the TARDIS console, pointlessly. "Apparently so. His bomb was meant to be a small and controlled explosion to break the drill for a short while. She'd be off shift then, when they broke through."

"What happened?"

He shrugs. "Natural gas released by the drilling ignited. Killed the Agent. Killed his sister. Killed everybody else in the control room of the facility."

Nine deaths. He can feel them, the rippling aftershocks of nine timelines suddenly snuffed out, consequences echoing back and forth throughout history. Gaps they must try to fill, now, to preserve the universe as he remembers it.

"That's terrible."

"Yes. Of course, the death toll could have been more. Were it not for the actions of an unknown rescue worker who didn't evacuate under orders, but went back to free several trapped schoolchildren."

She makes an odd expression, half frown, half smile. Pride, he suspects, battling humility on her face. "Is that me?"

"Records do not preserve her name," he answers gravely.

"Probably for the best."

He taps on the console again, wondering how best to phrase his next statement. "Clara, you could have died today."

"I know."

"That doesn't scare you?"

"Of course it scares me." She is frowning now, standing up out of his chair. "Doctor, what's the matter?"

"Choices," he says heavily, turning to look at the rotor as she comes to his side. "What you did today was selfless, brave and kind. No less than I expect from you."

"Thank you," she says, touched by his compliment.

"You do the right thing," he continues, seeing she has missed the point, "Even if it kills you."

"So do you."

"It's different for me," he says, waving away her returned praise. He can regenerate. He has lived for over two thousand years. He is alone; the last of his species, bar one megalomaniac he's less than keen to reconnect with. "One day… one day-"

"I won't be so lucky," she finishes for him. "I know. And that's my choice."

"Because I put you here," he says, finally at the crux of the matter.

She laughs. "Is that really what you think?" She leans against the console and takes one of his hands, drawing him to face her. "Doctor, you came to find me, the real me, because I scattered copies of myself up and down your timeline. I hate to break it to you, but you have never 'put' me anywhere. I'm here because of me." She sighs. "I'm like a fixed-point, Doctor. I made myself an inevitability."

He folds his fingers over hers. "Yeah," he agrees, smiling slightly, "I suppose you did."