Unlike the other pens, Porthos's had steel bars running over the top of it in a tight grid, about ten feet above the level of the water, leaving room for the whale to come up and breathe or roll over or do whatever else orcas felt the need to do at the water's surface. Except jump - he'd have to wait until morning when he was let out into his main pool to do that.

"How come only he's got the cage?" wondered Tony. "I mean, it's not like he's going somewhere. Whales can't come out on land, can they? Like seals? Seals can but whales can't, can they? I've never seen one do that but, come to think of it, I haven't spent much time studying whales. If they can that'd be cool."

"They can't," Alexei assured him.

"Then how come Porthos has got the cage? They think he's going to jump up and have a keeper for breakfast? That'd be boss. I mean, terrible, too, but... you know. How come he's the only one with a cage?"

It was possible that the bars were at least partly for the safety of the keepers - orcas could be dangerous - but I had another theory. "To stop people getting in.”

"Getting in? You mean, people who want to swim with whales? People do, you know."

"To stop animal rights activists," I said, hastily adding, "like us."

"Yeah, right." Tony wasn't remotely buying that anymore.

"The whales are the ones people worry about being in captivity the most," I said, suddenly feeling a little different about this whole enterprise. "They're too big for pools this size and too smart to be kept somewhere so blank and boring." I needed to focus on what I was doing and eyed up the computer-controlled lock on the trapdoor that led through the bars. "Can you open that?"

"Of course, I can."

As Tony swung his laptop into action, a thin, baleful sound emerged faintly from the tank beside us.

"What the hell was that?" asked Tony.

"That's Porthos," replied Alexei.

I knelt down and stared into the clear water. Below the surface, I could see the huge shape of the whale, slowly drifting around the limits of its pen, circling a path it must have taken so many, many times before.

Another soulful and haunting sound drifted up through the water, cutting through me with penetrating sadness. I felt as if I'd been slapped. Maybe I had something to fear from this creature that could, if it wanted, bite me in half, but it had a hell of a lot more to fear from me. Look at what my species had done to it. No wonder Poseidon hated us so much - look how we treated his creatures.

"Shit," murmured Tony, his hand frozen on his keyboard. I think it was the first time I'd seen Tony speechless, or so distracted that he couldn't even work.

"The door, Tony," I said, quietly.

Tony shook his head and went back to work. The red light on the lock flashed for a moment and then turned green.   

"S'open," Tony muttered.

I nodded and looked down at Porthos again, still making slow, lazy journeys the short distance around the pen. So small for an animal of that size. Were whales gregarious animals? I didn't know enough about them to be sure but I had a horrible feeling they were, and it was hard to imagine an animal of that intelligence not being lonely, on its own for so long. I remembered my dream and how relieved I had been to wake from it. Porthos was living his nightmare, one from which he never got to wake up.

Why should I feel so emotional over an animal? Why should Tony? I looked up at Alexei. There was something in his face I had not seen before; I wouldn't call it disappointment, but for the first time since we had met, I felt like he understood why the gods were so angry with our race, and I bristled with shame.

"Okay," I said, trying to clear my head of all these unhelpful thoughts that had trespassed into it. However sympathetic I was to Porthos's plight, I couldn’t accept that the extermination of the human race was a proportionate punishment. We weren't all bad and we could learn to be better.

I stripped off my sodden clothes to reveal the practical one-piece bathing suit I had on underneath - I had found it in one of Dolos's wardrobe rooms and was trying very hard not to think about who it might have originally belonged to. I passed my clothes to Tony.

"Hang these up, maybe they can dry a little."

He nodded and I sat down on the edge of the trapdoor, a retractable ladder leading down to the water beneath me.

"Cat." I looked back up to Alexei. "Be careful.”

I knew what he meant. It was so easy to sympathize with Porthos, so easy to listen to that sad song and feel his pain, to watch his endless circling and pity him his fate. But he was still a wild animal, one that had no such empathy with me, and which might be hungry. There was a reason that they were called killer whales.

"I will."

With that, I climbed down the ladder and slid into the water. I didn't want to startle Porthos but I also didn't want to creep up on him, so I stayed at the surface, treading water and letting the ripples spread out from me, alerting the whale to my presence in a nice, unthreatening way. I hoped. Clearly, he had noticed me since he had ceased to circle the tank and now hung in the water at the opposite end. I could feel him watching me, sizing me up, perhaps wondering what I wanted.

It occurred to me that when keepers and trainers came in here they presumably had some sort of regular routine - maybe they rewarded him with treats. There was no way I could emulate that. Was that likely to make Porthos pissed?

I had to swim by his side if I wanted to complete the task - it had been quite clear about that. So should I go to him? It felt safer to wait for him to come to me in his own time rather than forcing the issue, but I could be waiting a long time, and with two unconscious guards and the possibility of a shift change at some point, time was not my friend. Then, just as I had more or less made up my mind to take the initiative myself, the animal began to move towards me.

I forced myself to be calm, regulating my breathing, trying to time it with the strokes of my hands and feet as I continued to tread water. But fear was rising in me. Porthos's song might have spoken of a sad and lonely soul, but the sight of that sleek, powerful animal, sliding towards me through the water, knowing that it could tear me limb from limb, was a heart-stopping one. I held my position as he drew closer. The water that supported me now seemed to be tightening on me in my nervousness, like a second skin. Before he touched me, Porthos edged to my left instead, describing a slow circle about me, taking me in to decide if I was friend, food or threat. Around he went again as I tried to remember to breathe. And once more. Then, as he came up behind me one more time, I felt the water stir and, a heartbeat later, felt his blunt snout bump gently up against my back. Then, nothing.

Still moving slowly, making no sudden movements, I turned in the water to face the animal. He looked so much larger than he had a moment ago, but also, with his head protruding from above the water, so much more real. Looking at him beneath the water had been almost like watching a whale on a TV screen, but now that he had joined me in the air, he seemed more there. With a trembling hand and a fluttering heart, I reached out and stroked his snout, just in front of his blowhole.

Stroking a whale isn't like petting a dog or a cat, you don't get a wagging tail or a gentle purring to let you know they're enjoying it. But, as far as I was able to ascertain, Porthos liked being stroked - he didn't kill me, which was a good start.

He dipped his head back below the surface and once again I heard the sad, keening strains of his song. Without hesitation, I stilled my kicking legs, and dropped down with him. Now, I had joined Porthos in his world and, to my delight - and relief - he seemed keen to welcome me to it, swimming up to me and nudging gently against me as I ran a hand across his smooth skin. His song continued. It was the most beautiful thing I had ever heard, quite indescribable in its melancholy. I know it's easy to get carried away ascribing human emotions to animals, but with Porthos, the force was irresistible and the conclusion unavoidable. His behavior backed up the lonely cries of his song.

I rose to the surface to grab a brief breath then dived again, this time feeling bolder. I swam up to him, taking his head in my hands and resting my forehead against his snout. For what seemed a short infinity, we stayed like that, suspended in the water, head to head, two very different animals, finding a moment of touching commonality. I don't know what, if anything, that magnificent creature took away from that moment, but I felt as if my whole world had changed within it.

As he swam away, I accompanied him awhile, by his side, fulfilling the obligations of my task, though it felt somewhat hollow now, and I felt strangely guilty to do it. As I struck out for the surface, knowing that I could not linger here any longer than I already had, I looked back to see Porthos swimming in a strange, new pattern as he sang, as if executing a sad dance to say goodbye to me. Maybe I was reading too much into it, but I waved back.

Reaching the surface, I grabbed the bottom rung of the ladder and hauled myself up, thoroughly shaken by what I had experienced. The gods weren’t totally wrong about us. We had failed our world in so many ways. And just saving the earth wasn't enough anymore.

I had to figure out how to save Porthos, as well.