"Nobody's going to survive this."

"Yes they damn well will."

You reach out to me and place your hand on my shoulder. I tense up, unwilling to feel its comforting warmth.

"I get it, sweetie," you say. "We were close. But the first asteroids are due to hit in two hours. Let's make that time count."

If there had been anything in the next two hours—a sunrise, a sunset, even a thunderstorm; something to punctuate humanity's time on Earth—your suggestion might have held some sway. But, no. It's the middle of the day, and a cloudy one at that.

I finally look up from my work, feeling your gaze upon my nape. You're sad; exhausted; but there's a flicker of hope that I might come away with you. It breaks my heart to tell you otherwise.

"I can make this work. There's just a couple of fixes left."

You sigh, the glimmer in your eyes gone. "On what? The genotype selector? The spline resonation? The power spikes? What makes you think you can fix them all now when the whole team didn't manage it in the last year?"

A year hadn't been a long time for humanity to adjust to the idea of almost certain extinction. Sure, there would be pockets of survivors, even after the more long-term effects of the impacts. There might even be some minimum viable populations afterwards.

Some people were still clinging to the hope of surviving, digging great underground shelters that might even protect them from the smaller asteroids. Others had thrown themselves into wild hedonism: rioting, fucking, or smashing their brains out with drugs. Or a combination of the lot. I mean, I certainly had for the first couple of months. Before I'd met you.

Before you'd headhunted me, you'd jokingly correct me every time I brought it up.

Bloodbots was the latest fashionable name for this project and those like it. Programmable cell-sized mergers of electronics and biology. Pretty unstable stuff, but I'd been good with the prototypes a couple of jobs ago. Apparently people with this skill set were hard enough to come by, so that, as of the winter just passed, "a couple of jobs ago" was good enough to get me on the payroll.

It wasn't the salary that brought me in, of course. The company obviously had nothing to offer me—what use was money now?—but while you'd been pitching me the job, during informal meetings across a week while I'd sobered up, we'd become captivated with each other. Forget about secluded islands, personal safety, philanthropy: I'd come here for you alone.

But I soon realised that humanity could make its way through this. Hell, even the dinosaurs managed to survive their own extinction-level event. They'd just had to evolve a bit first. I glance outside through the small window behind you, where some gulls strut along the top of the sand dune as proof of it.

This particular company didn't have the tech to genetically alter humans (I don't think any of them are still around), but we could augment them.

"I have to try," I lie. There was no way the bots would be perfect, but I wasn't aiming for that now.

You can always tell when I'm not being honest with you. Your expression hardens a little. "I won't let you release the bots unfinished. The resonation problem alone could wipe out humanity, or whatever's left of it."

The data on my screen proves you right. These bloodbots will self-replicate, spreading like a virus around the world and altering basic human functions, making people more resistant to radiation, able to regulate their body temperature better in extreme weather, and a whole load of other post-apocalyptic survival traits the team had brainstormed at the start. But with the resonance still this high, it would stop cell development in almost all humans it targeted, those over age five or so, depending on the individual. It would be a plague.

I could fix the resonation problem, though. Get the affected population down. It was easy enough when you ignored the other two bugs. The genotype thing just meant that some people wouldn't get the "virus", which given the undesirable effects of the power spikes might turn out to be a good thing. This particular bug, by chance, happens to cause the bots to reject people with Rh-positive blood types, so we'd get a pretty effective randomised control test going on here. If the augmented humans managed better, great. If not, we still have the unaffected hypothetical survivors.

You see it differently. We've argued about this before, and it still irritates me.

"Oh, that's right," I reply. I can't stop myself. "I forgot you'd rather humanity died out."

"You and I are going to die for certain," you snap back, correctly: no bot will help us breathe underwater when this small island is submerged. "So why do you care?"

"I like humanity," I turn away to look at the simulation results. I've got resonance down to nine point seven percent. I could get that down another five points in the next hour. I bring up the network and make a couple more alterations, trying to think of something more convincing to reply to you with.

"Since when?" you fire back, exasperated. "We have trashed this planet in so many ways. You only started caring about saving everyone when you realised it was something you could take the credit for. I bet your name's plastered all over the code for someone to find later."

Ouch. It wasn't plastered, but it was there in a couple of headers.

You misinterpret my silence for irritation, and cry out in frustration. "I am not doing this again. We've got less than two hours now. I am going to spend it outside in the open air."

"Fine," I growl, and look back to watch test results scrolling up the screen. I hear you slamming the door behind me.

I have to keep working; I'm close enough to having a panic attack as it is without thinking about my impending drowning. But my mind is drifting…

There was a tsunami in Hawaii during the twentieth century; everyone raced to the beach where the water had receded far past low tide level; it was quite a novelty. They hadn't realised it was the effect of the sea drawing up into a tidal wave. Nearly everyone died. Nobody would make that mistake this time around; there had been so much analysis of what was going to happen in the media. Still, my mind churns over and over, trying to think of ways you and I could survive. Maybe we could use one of the lifeboats here? We might get lucky and not be swept under, or run out of food and water…

The tests ping again, bringing me back to the lab. Down half a point, to nine point two percent. Good. I pull up the network and start work on the next round of improvements.

Am I just doing this out of self-interest?

You come back half an hour or so later, your eyes red.

"I don't want our last memories to be of us fighting," you say quietly. Realisation strikes: I've been an idiot. You're as scared as I am. I get up to hold you, and break into tears. It doesn't take long for you to do so too.

"I'm sorry," I sob.

We sink to the cold lino floor in each other's arms, unable to talk.

After our raw despair passes we make love on the staff sofa, the only soft surface in the room. Things seem better after that, cuddling up together on the far-too-thin seat.

Eventually we decide to go outside together and face whatever the world is about to throw at us. We don't bother getting dressed. The rest of the staff here left weeks ago.

I glance at the terminal on the way out: my final simulation before you returned has finished with resonance of just under six percent.

It'd do. Some people will die from it, maybe a tenth or so, but it's low enough to stabilise after a few generations.

You fill a glass of water from the sink on the far wall. I switch to the emergency console, already fired up and authenticated since yesterday afternoon. One command to send the networked code to the dozen manufacturing hubs around the planet, and a second to cue up the release of the bloodbots when they were ready, about fifteen minutes later.

But first, a quick search and replace: my name removed. If this saves the human race, nobody will ever know who I was.

I don't know if you saw me doing it, but you say nothing, instead going to the door, and beckoning me outside.

We climb over the sand dunes separating the concrete office from the beach, and run down the embankment just like we did when I first arrived here, flown in by a helicopter with blackened windows. We play chase until you catch me and we fall to the sand, laughing.

By some miracle, the clouds clear, causing the brilliant blue water to sparkle and the sand to shimmer. We lie, content, looking up at the sky.

The first asteroids are now bright enough to be seen even with the sun out. My breath catches in my throat. They're both beautiful and terrifying.

"What do we do now?"

The warm sun is already inviting us to stay on the sand. It seems as good an idea as anything. We take each other's hands in silence.

The first asteroids, now blazingly bright disappear over the horizon. Less than a minute later the ground rumbles as we feel the impact of at least two separate hits.

Seconds later the beach stretches out in front of us, exposing the rocky sea floor. Your grip on my hand tightens, and I squeeze back. I look over to you; we are both terrified.

"I love you."

"I love you too."

The tsunami hits.