Galen finished taping the explosives to the inside the pump casing and breathed a sigh of relief. It was only half-full, even with the detonator in place – plenty of space for the radioactive payload, and yet still small enough to fit in his tool bag. If he wanted to destroy the city, all he had to do was put all the uranium rods into the casing beside the explosives and when the whole thing detonated...the city would be nothing but a radioactive crater. Not a single soul among those locked in the Colony would survive. It was a sobering thought.

If he'd known nuclear weapons could be so small, he wouldn't have been so surprised Humans had blown up their own planet with them. In fact, he was stunned that they'd lasted as long as they had. But he had to put this somewhere a siren would place it, not a Human, to make it look like it was Halcyon's work. That meant underwater. He knew just the spot, too – where the water supply bubbled up under the Aqua Dome from the springs deep below New Hope's surface. Without a set of Mer gills and a tail, he'd have to use a rebreather mask and fins, but he'd found plenty of those in Storeroom Five during his stocktake.

Someone tapped him on the shoulder.

Galen pulled off his headphones. "What?"

"How's the new satellite doing?" Alpen asked.

Galen stared at the grey-skinned incubus. "It's not a satellite."

Alpen shrugged. "We've all been placing bets on what you're building. One of the other guys thinks it's some sort of anti-surveillance device, so we can have strippers in the workshop without the Watch knowing. Another one said it's a still for brewing alcohol. I figured it was a communications satellite so you could sell black market communications to the rest of the system. Guess I was wrong. Unless you're lying so you can keep all the call fees for yourself. You'd be rich by the time you got out of here."

"It's not a satellite," Galen repeated, turning off his music player. "It's a water purifier. Some of the residents in the Aqua Dome have reported contaminants in the water, so I'll be putting this in the pipes below the dome to see if it can fix things, or at least remove enough contaminants to stop the complaints." That was the cover story he'd thought up and he was sticking to it.

"What sort of contaminants?" Alpen asked. "I remember one of the ships I was working on had this hideous stench to the water supply..."

"Don't know," Galen interrupted. "Can't detect anything out of the ordinary, but the Mer insist there's a problem. This is me trying to fix it." He waved his hand over the pump casing.

Alpen shrugged. "Good luck. Unless you need anything, I'm done for the day. Heading home for a meal. If the HVAC system has any hiccups you can't handle, comm me."

"It'll be fine. See you tomorrow." Galen watched him go, then donned his headphones and switched the music back on.

Allie had suggested they go to a fairy bar tonight, but she'd called to cancel an hour ago, saying she'd gotten caught up in a repair job over in one of the residential domes, so they'd have to reschedule for tomorrow instead.

Knowing he'd almost finished the bomb, Galen had decided to bring the schedule forward and put it in place tonight, so he could celebrate with Allie properly tomorrow. He'd decided not to detonate it for a few days. Maybe he'd even set it off tomorrow, while he was on that date with Allie. He'd have a solid alibi then. Not that he should need one, but you could never be too careful.

He'd originally planned to plant the bomb late at night, when no one was around, but he'd since realised that would look far too suspicious. Instead, he carried the modified pump in one box and the uranium rods inside his toolbox, looking for all the world like a plumber about to go replace a pump. Routine maintenance.

Sure enough, no one in the packed aircar gave him a second glance. They glared at his burdens as he squeezed in, before shuffling aside without a word. It was a long aircar ride to the base of the Aqua Dome, but by the time he reached it, Galen was the only passenger. Once again, he wondered how the Mer got around the Colony. Did they use water-filled aircars, or did they stay in their aquatic habitat, avoiding the rest of the population? He hoped it was the second choice. Allie had pointed out different Titan races to him when they met for dinner in the evenings, and he now prided himself on being able to identify at least a dozen. She'd never pointed out a Mer, though, for which he was glad, because he wasn't sure he could hide his disgust at a species that killed so indiscriminately. Because Halcyon couldn't have been acting alone. She had to have had help, allies, maybe a whole Mer army to create the kind of body count she was responsible for. Galen still found it hard to believe that only one Mer had taken down a ship the size of the Poseidon. She must have had a team of them then, just like she'd have a team of them now. Someone to help her sneak onto the ship. Someone who knew its inner workings so she could blow it up. Someone who constructed the bomb for her in the first place, because he knew there was no way he could have built one underwater, and no siren could beat him when it came to Human technology.

Of course, he'd done everything alone, but he was Human, capable of walking around in the air and working in precisely the right place to make and plant whatever he needed to. No Mer could do what he could. Not to mention he'd have noticed the moment one entered his workshop. He wouldn't need Allie to point them out – he'd smell them coming.

Not that there would be any Mer in the maintenance corridors beneath their dome, Galen thought with considerable relief as he palmed open the door to the restricted area. Only authorised personnel had access. A terrorist like Halcyon would probably bribe her way in, or threaten someone into giving her illegal access. At least, that's what he'd say if anyone asked him how she'd gotten in.

The sound of gushing water echoed through the corridors as the hatch crunched shut behind him. If he closed his eyes, Galen could almost imagine himself at sea in a lifeboat again, wishing for rescue in whatever form it came. Then, he'd even hoped the siren he'd seen would help him. Thank the universe she'd ignored his pleas for help and swum away, if she'd even heard them. Galen wasn't sure he could have lived with himself, knowing he owed his life to Halcyon.

He shoved through the memories and strode into the bowels of the Colony. Tonight he'd lay his trap to catch a siren, and soon he'd spring it. His parents' shades could rest, knowing they'd been avenged.

After what felt like forever, he emerged into the main pumping station. The spring erupted in a fountain in the middle of the cavern-like space, filling the room almost to the level of the catwalk where Galen stood. A labyrinth of pipework ringed the room, drawing water from the spewing source and carrying it ever onward and upward to supply the Colony.

Galen knew exactly where he wanted to place his package. There was a spot where a dozen inlet pipes converged, and something as small as a new water purification unit...or a nuclear device masquerading as one...might go unnoticed.

He still had an hour before the lights dimmed and the Colony's night began, though, so it was possible that someone could walk in on him. Unlikely, but possible. Galen took the uranium rods out of his toolkit, and nearly dropped them, they were so hot. He managed to stash them out of sight under the steps, hoping they wouldn't melt the metal before he transferred them over to the bomb. He had to work quickly, then.

He stripped down and donned a wetsuit, then perched on the edge of the catwalk to wedge his feet into a pair of fins. Galen left his toolbox open on the edge of the catwalk, within reach of the water, and slid into the depths. He bobbed on the surface for a moment as he adjusted his rebreather mask, tapping it until the lights glowed into life, before he submerged completely.

Underwater was a completely different world. No wonder Mer seemed so alien to him – they didn't even breathe air, or so he'd been told. He'd never gotten close enough to see one's gills.

Galen swam carefully around the fountain, which sounded thunderous under the surface. Visibility was low with so much turbulence, but he knew if he kept to the walls, he'd be able to follow the pipes to the nest of inlets. The pump casing hindered him at first until it filled with water, and then it was just an extra weight to tow along until he reached the spot. Just like the schematics showed, there was a perfect little alcove between three pipes, just big enough to fit the pump casing, but shadowed by the pipes overhead so it wouldn't be immediately visible. Plus, when it blew, the bomb would damage at least ten pipes within the blast radius of the explosives alone. It was a shame to destroy such an ingenious water supply station, but his team would be the ones rebuilding, so they'd soon get it back in working order, secure in the knowledge that no terrorist would target it again.

Galen had planned to fasten the casing in place, but it fitted so snugly in its alcove that it wasn't necessary. Nothing but the blast itself or a seriously strong water current would budge it. Now all he had to do was pop the payload in and he could be home before dark.

He kicked off the side of a pipe, but his fin caught on the join, so it was wrenched off his foot. Cursing, Galen reached for it, but in vain – his fin sank into the depths, deeper than he could go with just a rebreather. It didn't matter. There were plenty more where those had come from – dozens to spare in the equipment store at the end of the catwalk. He peeled off the other fin and set off in the direction of the steps.

It was harder going without his fins, especially now he was fighting the current, too. He felt the faint pull of the pipe inlets as he passed them, but that was actually a good thing, as it kept him away from the fountain. He made it halfway back before a particularly strong tug dragged him upward, into one of the pipes closer to the surface. Galen tried to fight it, but the current was too strong, pulling him along until his shoulders wedged into the pipe and he found himself stuck fast.

He fought to free himself, but the suction of a pump somewhere above him was stronger than any pressure he could exert, plus the water pouring into the pipe behind him, rushing to reach its destination, pushed him deeper in.

His rebreather only allowed him an hour of air. Past that, and he'd drown. He had to calm down and think. If he was wedged too tightly in the pipe to get himself out, he'd need help. He couldn't call anyone, so the next best thing was to trigger an emergency alert. The easiest way to do that was to block the pipe, and hope there was still someone up to answer the alert before his air ran out.

Galen spread out his arms and legs, trying to angle his body so no water could get past him. The current buffeted his back, trying to force its way through, but he held fast, fighting to block the flow for as long as his strength held out. His only hope was to trigger an alert.

After an eternity that was probably no more than fifteen minutes, Galen gave up. He hurt all over and the pressure was starting to make his head pound. Or maybe he was running out of air. He hoped it had been enough.

Something brushed his bare foot. Something warmer than the water. Galen twisted his head to try and see behind him. His headlight reflected off something orange, before he saw nothing but water. It couldn't be. He blinked, watching the pipe inlet again.

This time, he clearly saw a coppery tail fluke brush against his foot, before the rust-coloured Mer blocked off the end of the pipe.

Halcyon. It had to be. Galen tried desperately to squeeze inside the pipe so she wouldn't see him, but all he succeeding in doing was to bash his head so hard against it that he saw stars. Then the stars faded to blackness. Galen's last memory was terror at what the siren would do to him – a helpless Human caught in her world – before he lost consciousness altogether.