I

In an era when spacecraft were plentiful, and teleportation was cheap and easy, no one walked any great distances anymore.

On the surface of the moon, a boot stamped into the thick, dark grey dust that carpeted the seabed of the Ocean of Storms. It was worn by a tall man in an armoured military spacesuit, designed to protect its wearer from the most hostile environments in the Solar System and to do it while the enemy was shooting at him. The suit was triple-layered, reinforced and coloured dark grey, only a shade lighter than the dust at the man's feet. His helmet visor was tinted black. Five more figures, wearing identical spacesuits, followed him. A small warehouse robot, carrying a large open-topped crate on its caterpillar tracks, brought up the rear.

The Ocean of Storms was a vast open lake of the dark grey dust, reflecting less light than the rest of the lunar surface. A small mountain range rose on the curving horizon behind the figures, with the blue and white sphere of Earth rising above it, casting long shadows from the peaks out over the seabed. Around it, far out in space, the stars shone more brightly than they ever could on the surface of the planet with the atmosphere diffusing their light. But the figures didn't look back to admire the view. Their target was ahead of them on the opposite horizon.

It stood on a long, curving platform of rock that reached out into the Ocean of Storms like a peninsular. It was a collection of square and rectangular nine-storey buildings, made of dark grey ultracrete, with long windows glowing with the interior lights. There were no airlocks at ground level and the exterior walls were two metres thick. If anyone did happen to be looking out of the windows in the right direction, the six figures would be hidden against the shadows the mountains.

The lead figure paused beside a small rocky outcrop, the remains of some ancient meteorite strike, and checked his Unirifle one last time. The computerised sights were zeroed, so he could aim accurately through the curve of his visor, and the power cell was fully charged. The other figures did the same, then indicated with hand signals that they were ready. The leader nodded, then turned forwards and led the way towards ChronOps HQ.

_ _ _ _ _

Inside Chronological Operations Agency Headquarters, it was lunchtime. Mirabi and I were sitting at one of the long tables in the main canteen, waiting for the afternoon duty shift to start.

"I'm completely serious," said Jake Helios, who was sitting opposite Mirabi. "Is it possible or not?"

"Of course it isn't, you moron," said Jake's partner, Deborah Shiva, who was sitting next to him. Jake was my height, with blue-green eyes and short dark hair, while Deborah was shorter, with straight red hair that hung down to her jaw and light grey eyes. "We can't change the past. Ergo, we can't change the future. Same reason."

"Exactly the same reason," said Mirabi, eating another mouthful. My partner was slightly shorter than I was, with long black hair in a braid that hung all the way down her back. She was over-eating as usual, but still looked fit and athletic because she'd recently taken up low-gravity Taekwondo again. "If you want to change the past, you have to go back even further into the past - before the event you want to change - to do it. So effectively, you're trying to change the future. That won't work and there's no reason why it should if you do it from the present."

"It's never been tried," said Jake, pointing a half-chewed bread roll at her.

"Of course not. The only people who want a repeat of Mercury are the cultist nut jobs, the deluded and the stupid," said Deborah.

"That still doesn't mean we know for sure," said Jake. His main passions in life, aside from his ChronOps career, were 3D poker and sunboard-surfing, but he was thoughtful about the logic and consequences of time travel past the point that gave most people headaches. "The future's meant to be the Fan of Open Paths. Our choices in the present determine which one we go down. So why can't we look at the future, choose which choices to make and which path we go down? If we can't, that means there's actually only one path."

"Well, yes, but we can never be sure we haven't already made the choices that have led to that particular future," I said.

"OK, but suppose we could?" said Jake. "Then is it possible to change the future?"

"Well, possibly. But we could never be certain it wouldn't cause a paradox reaction," I said.

"Yes, so if you're going to try it, don't do it anywhere near me," said Deborah. Her own great passions were extreme sports and speed target shooting and, while they made a very good team, she had spent years being driven to distraction by Jake's musings on how the time stream actually worked.

"Don't worry. I'm not trying to lose my time flight license," said Jake. "But it would only cause a paradox reaction if it changed the future. That would mean there's only one future to be changed. That means the theory's wrong and it's the Fan of One Path, not the Fan of Open Paths. But time travel's already proven there's no such thing as destiny, so..."

"If you say 'but' one more time, I'm going to strangle you," said Deborah.

I tuned out the conversation a bit at this point. It was one of those arguments about time travel that was never going to come to any kind of conclusion. Just like time itself, you could go forwards or backwards, but you'd never come to an end. But more than that, I wanted to just enjoy the time, the place and the situation for a moment. I looked around the canteen, where dozens of our fellow, black-uniformed ChronOps officers were eating, talking, laughing and arguing. More were entering and leaving every few minutes. Three months ago, after our hair-raising trip to the Time Traveller's Ball on Deimos, I'd made an unauthorised trip into the future, to the 30th of September 3029. I'd learned there - to my immense, everlasting relief - that I wasn't destined to die on that day, trapped alone on a burning space station, as I'd believed nearly all my life. Since I'd gotten back, having learnt that I did have a life of my own to live after all, and one without a set expiry date to it, I'd been enjoying the simple pleasures of it - like having lunch with good friends - a lot more.

"OK. But what if..."

"All right! That's it!"

We were interrupted at that moment by the figures blasting a hole through the outer wall.

_ _ _ _ _

The crate the robot was carrying was filled with limpet charges. The intruders stuck six of them to the outer wall in a rough diamond, set the triggers and moved along the building, well out of the way and braced themselves against it, pressing their backs against the wall and digging their heels into the ground. The explosion was still powerful enough to throw four of them flat when the lead figure pressed the switch on the remote detonator.

The blast was silent on the airless lunar surface. A roughly triangular section of the wall disintegrated instantly. A huge cloud of dust was thrown up into the air in front of it and then was blown away as the room on the other side of the hole depressurized and its atmosphere rushed out into the lunar vacuum. The water particles in the air froze solid in nanoseconds. They fell like tiny snowflakes, landing on top of the dust blown smooth by the rush of escaping air, and glittering in the starlight. Seconds later, they were crushed underfoot as the intruders rushed forwards and ducked through the hole into the building, their Unirifles raised and ready.

By some small miracle, the room on the other side of it had been empty. No one had died from the blast or the decompression. The torch beams on the intruders' weapons swept the corners of the room - the blast had broken all the lights - and confirmed it was deserted. The long storage room only contained storage crates, holding solid records too valuable to be committed to computer. There were only two doors and, on the other side of these, they faintly heard the clamp of the supersteel emergency doors, slamming shut and auto-sealing, isolating the area around the hole and preventing the rest of the atmosphere in the building from escaping.

The leader kicked the storage crates out of the way and strode to the end of the room. The wall at the other end, opposite the hole in the outer wall, was bare and - being an interior wall - much thinner. The robot trundled up behind him and the leader took only one charge out of its crate this time. He slapped it against the wall, in line with the first hole, set the trigger and everyone stepped clear again.

_ _ _ _ _

Upstairs in the canteen, we were all thrown out of our seats by the shockwave.

"What the hax was that?!" said Deborah, pulling herself up on the edge of the table.

"Meteor strike?" said Jake, sitting up. The soup he'd been eating had gone all over the front of his tunic. Something wet slithered down my neck and I thought for a moment I was bleeding, but then I reached up and touched it and realised that most of my mushroom ravioli had landed in my hair.

"Everyone all right?" called one of the medical officers from the middle of the room. "Any injuries?"

"No. NG. I'm OK," voices called back from across the room. Mirabi and I stood up as well.

"That wasn't a meteor," she said. "The sensors would have detected..."

"No. You're right. It was too close," I said. "It felt more like..."

I was about to say it felt more like being under artillery fire, a pleasant experience we'd once enjoyed on Charon, when an alarm - one of the ones which should have started shrieking the instant an incoming meteorite was detected - started to sound. But this one had a distinctive, wailing tone that we all recognised.

"Atmosphere breach," said Deborah.

Everyone dropped what they were doing and ran out of the canteen, jumping over the tables and crowding out through the doors. We went with them, with the kitchen staff bringing up the rear. We all sprinted down the nearest corridor - like running in particularly crowded marathon - to where other people on this floor were already gathering around the nearest big wall screen.

"Oh, haxing wonderful," said Mirabi.

The screen was showing the floor map of the entire HQ complex. The rooms on the ground floor were shaded in blue, except for the one where the breach was, which was flashing in red, showing there was nothing to breathe inside it anymore. The three rooms around it were shaded in a lighter blue, showing that they were still pressurized, but in the danger area. The sealed emergency doors were shown in thin red blocks.

"Well, at least it doesn't look too bad," I said.

"Thank you very much for your assessment, Midgard, but I think we'll wait to hear from the experts," said a voice striding up behind me. I turned around as Deputy-Commander Eleanor Caelestis, one of ChronOps's tall, blond and very commanding senior officers arrived. "Hephaestus. Report."

"He's right, Ma'am. It's not serious," said Captain Duncan Hephaestus, the chief engineering officer, who was examining a boardcom. "I don't have a clue what caused it, but it won't take long to..."

The floor suddenly shook again. It wasn't as powerful as last time, but it was enough to break everyone's balance and make us stagger and grab handholds. Jake stumbled into Mirabi and I caught Deborah.

"Oh, Devious Darwin!" said Hephaestus.

The map on the wall screen suddenly changed. Another room, directly in front of the first one, going deeper into the building, suddenly depressurized and started flashing red. More red blocks appeared on its edges and the rooms around it turned the lighter blue as they sealed themselves too.

"Sorry, ma'am," said Hephaestus. "It must have done more damage than the sensors are picking up. This might take a bit long..."

The floor shook for a third time. It seemed sharper and closer than before, and I realised I'd faintly heard an explosion somewhere directly below our feet. Mirabi grabbed my arm to keep from falling and several of the younger male officers leapt forward to catch Caelestis.

"What in the name of evolution...?!"

Hephaestus shook his boardcom and then banged it twice against the wall. He looked up at the big screen and blinked as it confirmed what he was seeing. A third room, in a straight line from the first two, had turned red. The group of sealed rooms had extended around it too.

"What in Darwin's name is going on?" said Caelestis, who'd disappointed all the younger male officers by keeping her balance and staying on her feet.

"It's impossible," said Hephaestus. "This is impossible. No meteor can behave like that. It's not..."

"Get a grip, Captain!"

"Do we have any eyecams down there?" I said. A very tiny, very unpleasant possibility had just formed at the back of my mind. I shook it away quickly. I was being ridiculous. Because that was impossible.

"What? Oh, yes, of course. Hold on," said Hephaestus. He typed briefly on his boardcom's touch screen surface.

"Slightly better, Midgard," said Caelestis, as the view from the security eye in the latest depressurized room flashed up on the big screen.

Everyone fell silent. Even the whisper of the air cycling system seemed to stop. Inside the room, six figures in spacesuits were tossing boxes away from the wall at the end, so one of them could place what was obviously a shaped charge against it. They all moved clear and it exploded, blowing a hole straight through the wall, big enough for them to walk through without ducking. I felt the shockwaves ripple up through the floor again. The map flashed up in a miniaturised box in the corner of the screen and we saw a fourth room turn red. The figures in the spacesuits - who I now realised were all armed - hurried through the hole followed by their robot. The last one paused in the hole, turned around, raised his Unirifle and used it to shoot out the security eye. The main part of the screen turned black.

"Darwin, Newton, Einstein, Hawking and Dawkins," said Caelestis. She pressed a button on her wristcom. "All stations. General alarm. Intruder alert."

_ _ _ _ _

"Who the shav, who, in the entire Solar System, is stupid enough to hit us here?" said Deborah, pulling the power cell out of her Unigun and slapping it back in for the third time.

"If it's Titus and Ricardo, they're dead," said Mirabi, who was doing the same.

"Whoever it is, they've thought this through," said Jake, looking at the screen.

I could see what he meant. The map was back up and there were now six red depressurized rooms in a straight line, driven like a spear into the heart of ChronOps. They were surrounded by the halo of light blue rooms, which still had atmosphere, but - because of their sealed emergency doors - we couldn't get into them.

"We can't, ma'am," Hephaestus was saying to Caelestis. "The doors seal automatically in the event of a breach and then they lock. I can't just do an override. They won't unseal until the breach is closed and the rooms on the other side are repressurized. We can't get them open. Even if we could, we'd lose all the atmosphere in the rest of the building. They're blowing through the walls so they don't have to worry about the doors."

"All right. How long to seal the outside hole and repressurize?"

"...Four hours, probably. Minimum."

"Too long. Think harder," said Caelestis. She turned to one of the other officers who was listening. "In the meantime, get a dozen men suited up, issue heavy weapons and cutting equipment. Can we go inside a room and seal the emergency doors ourselves?"

"What? Uh... Yes. Yes, that's feasible," said Hephaestus.

"Good. We'll make our own airlock," said Caelestis. "Seal ourselves in, cut through the walls to the rooms they've penetrated, No loss of atmosphere. Start choosing a suitable room."

"I'm trying, ma'am," said the chief engineer, looking between his boardcom and the screen, just as the floor shook again and another rooms turned red. "They're just moving so daxing fast!"

"Can you pull the map back?" I said. "Can we see where they're going?"

Hephaestus tapped his boardcom. The map enlarged as the view zoomed out and we could see the whole of the ground floor plan, with the line of red rooms spearing through it. The floor shook again and it reached eight red rooms, going in a straight line towards a very large, circular room in the centre.

"Oh, Gentle Darwin," said Deborah. "They're going for the vault."

_ _ _ _ _

"Suits. Weapons. Cutting lasers. Move," said Caelestis.

She and the dozen officers she'd just picked - along with nearly everyone else - ran off towards the stairs. The lifts automatically shut down in the event of an atmosphere breach to maximise the power available to the life support systems. Hephaestus ran off in the other direction to join his disaster management team in the control centre. The expressions on all their faces us told that everyone knew exactly how bad this was.

The vault contained everything that ChronOps found in the course of our work as the Solar System's universal temporal police force; and particularly the things that could never be released - or even shown - to the public. Objects from the past, the future - and some that were possibly from parallel universes or alternate timelines - were stored there. Things found in places where they couldn't possibly have been, items that shouldn't possibly exist and technology that we could barely understand, let alone control, were stored in it behind ultratanium walls and sophisticated locks specifically designed to keep them in. But we'd never stopped to consider if they'd be enough to keep someone who wanted to steal them out.

"It's going to take too long," said Jake, shaking his head, as an eighth room on the screen turned red. "They're not going to be able to get ahead of them in time. They'll going to have to try and catch them on their way out."

"Out?" I said. In the back of my mind, something that'd I only been half-aware of suddenly clicked. "Jake. You're a genius."

"Thank you. Nice of you to finally notice," said Jake. "Erik?"

He looked around and realised I was already halfway down the corridor with Deborah and Mirabi.

"Erik. Talk. Where are we going?" said Mirabi.

"How are they going to get out?" I said, as we sprinted down the corridor and Jake ran to catch up.

"Same way they came in,