"But why would someone want to pa.s.s on a message in secret?" I asked. "If he was obsessed, why not tell everyone?"

"The only reason I can think of is so Neptune would never know."

"But why not act on it, do something about it?"

"He probably didn't know what it meant any more than we do. But he knew it meant something. Look." Aaron pointed at words scrawled all over the walls, painted around the pictures as though revealing the inner workings of the artist's mind. Why? What is the significance? How many years? the words said.

"Someone's been asking all the same questions as we have," I said.

"And clearly had about as many answers as us," Aaron replied flatly, "or else we wouldn't be in this position now."

I stepped forward to study the first painting more closely. It was only then that I noticed the shadows in the sky. The swirling patterns looked familiar. A dark, spinning cone in the sky.

Aaron moved to the next picture and motioned for me to follow. It was similar to the first. The same boiling sea, the sky even darker this time, the moon shining a reflection on the wet rocks like a beam from a flashlight. The swirling shapes were there in the sky again. One looked like a spinning beehive, another like a dark trail from a plane that had been looping the loop.

"There's one more," Aaron said, pointing to the third picture. It showed gray rocks and the base of the castle. The swirling patterns were now just one thick black swarm: a whirlwind, its base at the tips of the rocks, in the center of a shining white circle of light.

"I've seen these shapes!" I blurted out, suddenly realizing that the image had stayed in the back of my mind ever since we'd seen it. "The first night we were here! What are they?"

"I've seen them too. Usually at this time of year. It's birds. They come in the millions."

"At this time of year? The spring equinox? But, Aaron, that's proof! They must have something to do with the rings! And your great-grandfather knew it too."

"I think you're right," Aaron said. "But the thing none of us knows is, what are they telling us?"

I wasn't aware of whether he said anything else. I was too busy staring at the words I'd just noticed among the rest, in capitals and underlined, like a t.i.tle for the paintings.

My eyes glazed over, cold shivers running like electricity up and down the length of my body as I read the words: THE STARLINGS.

I don't know how I got through the rest of the day. Shona and I scurried away every chance we got, to talk about what I had to do and how it was going to work out.

We swam around the lower half of the boat.

"OK, so you have to get to the castle, find the other ring, and bring the two of them together," Shona said, going over the plans for what felt like the twentieth time.

No matter how many times we repeated what I had to do, it wasn't sounding any easier.

"All in the minute that the moon is completely full," I said. "Or it'll be too late. Neptune made his message clear enough. When the moon is full, the curse on me will be complete. I won't be a semi-mer any longer. And that means I won't even be able to touch the ring. I'll lose it forever." Along with everything else I care about, I added silently.

Shona looked at me, holding my eyes with hers. "Let's not think like that," she said.

"I'll lose a parent," I went on, ignoring her.

"Emily, please don't."

"And Aaron will be an orphan."

"Emily!" Shona took me by the shoulders. "Concentrate. We can do this, OK?"

"OK," I said lamely. I didn't believe for a minute that we could. The odds were just stacked so high against us.

The sun had set and the moon was up. This was it. A few more hours and it would be fully risen.

Millie wouldn't leave us alone. She stood on the front deck, pointing out the constellations as the stars appeared, one by one, across the vast sky.

"There's Canis Minor," Millie said, pointing at a clutch of stars that looked pretty much exactly the same as all the others. "And, oh, I think that might be the Corona Borealis." She consulted her book, then looked back up at the sky. "Yes, I think it is," she went on, oblivious to whether anyone was actually listening. "Well, you don't often get the chance to see that," she said.

I smiled politely at her when she called me over, making all the right noises so she'd think I had some idea of what she was going on about. All I actually cared about was how I was going to get away from the boat before the moon was at its peak. We couldn't risk telling Millie. She might try to stop us, and there was just too much at stake. I glanced at my watch. Nearly ten o'clock. Two hours. I couldn't even jump over the side and sneak away, as she didn't seem to want to leave me alone, let alone go inside.

I tried feigning huge yawns in the hope it would catch on and make her sleepy.

"Why don't you go to bed if you're tired?" was all she said.

Shaking my head in despair, I went to find Shona.

"What are we going to do?" I asked. "I can't get away while she's out on the deck. We're going to run out of time."

"Why don't you just tell her what you want to do?" Shona asked.

"I can't. She's already said again this evening that she's not going to let me out of her sight. I'm not going to chance it. If only I could hypnotize her or something, like she does to other people."

"Hey," Shona said, a slow smile creeping across her face, "I might just have an idea."

She rummaged around in her schoolbag. "Ta-da!" she said, producing her best B&D hairbrush. The handle was made from bra.s.s and was cast in the shape of a sea horse; the bristles were soft and feathery. On the back, there was a mirror surrounded by pink sh.e.l.ls.

"A hairbrush?" I said. "Shona, this is no time to worry about how we look! We've only got a couple of hours!"

"I'm not worrying about how I look!" Shona said crossly. "Listen. I've got a plan."

As she explained her idea, I couldn't help smiling too. "Shona, you're amazing," I said. "It might just work."

Millie was only too happy to oblige when I asked if she'd hypnotize me. "It's just a" I'm so tired, but I can't get to sleep," I said. "I need something to help me. I think your hypnotism is the only thing powerful enough to do the trick."

She giggled and blushed. "Oh, stop it, sweetie," she said. But she flicked her shawl importantly over her shoulder as I followed her into my bedroom.

I glanced at the chair I'd set up for Millie by my bed, hoping she wouldn't move it. It was perfectly positioned, as was the hairbrush on the dressing table. As long as she sat down without moving anything, the mirror should be in exactly the right spot for it to reflect her hypnotism back onto her.

"All right, then," she began, settling herself down in the chair. Perfect! I lay on my bed and half closed my eyes. "As you know, this is a powerful tool, so you may find you sleep even more deeply and soundly than usual," she said. "And you may find your dreams are more intense or elaborate. Don't worry about any of this. All that matters is that you have a good, long rest. Now, make yourself nice and comfy and we'll get started."

I fidgeted around for a moment, trying to act as if I were getting myself comfortable. All I hoped was that I wouldn't get too comfortable and fall asleep.

What if it doesn't work? a voice in my head wouldn't stop asking. I did everything I could to ignore it. It simply had to work. There was no alternative.

Moments later, Millie was drawling in a deep, low voice about how tired I was getting. "Imagine you are a feather," she intoned, "falling gradually down to the ground. With each breath, you sway a little bit lower, getting closer and closer to sleep."

I couldn't help yawning. Don't think about the feather. Don't think about sleep, I urged myself. Think about what you have to do. Think about your mom, about your dad, about your one single chance of getting them back together. And of not having to leave half of your ident.i.ty behind.

That was all I needed. I was wide awake. And panicking so much it felt as if a high-speed train were racing through my chest.

"You're sleepy," Millie drawled even more slowly, "very . . . sleepy . . ." Her voice was starting to sound as if she were drunk. "In fact, you are so . . . very . . . sleepy . . . that you can't even . . . think . . . anymore." She took a deep breath and yawned a very loud yawn before continuing. "All you want is to go to sleep"a" a long pause a""beautiful sleep . . ." An even longer pause. She yawned again. "Peaceful . . . deep . . ."

This time the pause stretched on and on until a brief snort erupted through her nose. I waited a few more moments before daring to open an eye.

I had to clap a hand over my mouth to stop myself from bursting out laughing. Millie lay sprawled across the chair, her legs spread out in front of her, head thrown back, mouth wide open, eyes closed.

I quickly sat up on my bed. Carefully edging past Millie, I crept to the trapdoor in the middle of my floor and lowered myself down.

"We did it!" I whispered excitedly to Shona. "She's completely out."

"Swishy!" Shona grinned. "Come on. Let's go."

We swam to the porthole and listened one more time. Nothing. This was it, then.

"Wait," I said. My tail hadn't finished forming. It was taking longer and longer. My legs had stuck together, but there were hardly any scales. I couldn't feel my legs a" but I couldn't feel my tail either. It was as though there were nothing there at all. The whole bottom half of my body felt completely numb.

For a second, I panicked. What was going on? Had I become paralyzed? Maybe I'd never walk or swim again!

Eventually my tail formed, what there was of it: bluey-green, shiny scales at the ends, fleshy white skin almost all the way down to my knees. It felt wooden and inflexible, flicking halfheartedly in the water. My breathing was raspy. I don't know if Millie's hypnotism had anything to do with it or if it was just the curse, but by the time we swam out through the porthole, I was so exhausted I could almost have fallen asleep in the water. My mermaid self was disappearing before my eyes a" and taking my breath with it. I was becoming more and more of a nothing, more and more of a no one. I didn't fit in anywhere. I wanted to give up and cry.

Shona swam ahead, her tail splashing shiny droplets that sparkled in the moonlight as she swam gracefully along. Would I ever do that again? Not that I ever swam as gracefully as Shona, anyway. My heart felt as heavy as the rest of me.

"Wait," I called, struggling to catch my breath.

Shona slowed. "We've got to hurry," she said. "We don't have long. The moon'll be at its peak within the hour."

"I know. I'm doing my best. I just . . . can't . . . keep up," I gasped.

Shona swam beside me and took hold of my hand. "Come on, Emily," she said softly. "You can do it. You've got me. We'll do it."

I didn't reply. No point wasting my limited energy talking.

But however hard we swam, the castle didn't get closer.

"Where's the tunnel?" Shona asked.

I shook my head. "No good," I said. "Can't hold my breath. Have to just swim."

I tried to do what I'd done the first time the ring had led me to the tunnel. Tried to let go, listen to the ring. I stroked the gold band as we swam and twisted the ring around so I could see the diamond sparkle and glint on my finger.

It was leading us there. I could feel it, even if I couldn't swim through the tunnel, even if the current was so slight I could have imagined it, even if the castle only seemed to be getting closer an inch at a time. Even if I didn't know how we'd find Aaron a" or the ring! We were still getting there, and the ring was doing all it could to help. Maybe it was getting weaker, like me.

Please hold out, I begged silently. Please get us there.

We seemed to have been swimming forever.

"I can't do it!" I cried. Tears were starting to slip down my face. "I can't do it."

"Emily, look!" Shona let go of my hand to point ahead. I followed the line of her finger. "The castle!" she said. "We're getting closer!"

She was right. My eyes keen against the darkness, I could see it more clearly than ever. The mist lay across its middle like a belt. Above, three large turrets stood proud, serrated against the deep blue night. Its windows shone as though polished, hiding a thousand secrets behind them.

Below the mist, rocks were emerging by the second. Huge boulders lay dotted about on the stony beach. In between them, jagged rocks were scattered everywhere, like a range of forbidding mountains. Waves thundered against them.

The sight of the castle so close spurred me on. I tried flicking my tail, but it hardly moved. My arms were weakening with every stroke, my tail growing more and more like a plank of heavy wood with every flick.

And then the moon was high in the sky. We'd have maybe twenty or thirty minutes till it was at its peak a" and at its fullest. It wasn't long enough.

"We're never going to do it," I said. "We might as well give up."

But before Shona had a chance to reply, a voice called across to us in the darkness. "Emily!"

I peered ahead, scanning the rocks.

"There!" Shona screeched, jabbing a finger at one of the huge, jagged rocks, sharp and pointed as a witch's hat, and just as black. A figure stood halfway up its side. Aaron!

"Emily! Hurry!" he called. "Please hurry!"

I couldn't give up! Of course I couldn't. It didn't matter if every single cell in my body wanted to scream with exhaustion. I had to get there.

Shona held tightly on to my hand. "We can do it," she said again and again. "I'm going to get you there." But pulling a dead weight along in the water can't be easy for anyone, and even Shona was starting to get tired. Still the castle lay out of reach. Come on, come on. We have to get there. Inwardly I urged myself on, screamed instructions and demands, begged, bribed. Just get there. I'll do anything.

The moon climbed slowly upward, growing whiter by the second. Any moment now, the curse would be complete and it would all be over. Neptune would be here to claim his ring a" and he'd be bringing my parents so that one of them could say good-bye. My chances of solving all this would be lost forever, along with everything I cared about. Including the chance to help Aaron.

I splashed through the water, clumsy and awkward, like a puppy in a lake. Useless. Useless! The castle seemed to be getting farther away. The moon shone down, its beam like a searchlight across the top of the ocean. I kept my eyes on the water ahead of me, hiding from the shaft of light like a fugitive. If it didn't catch me, maybe we would be safe.

I glanced up at the castle. Still too far. It looked like a cardboard cutout against the night sky. A silhouette, the little figure of Aaron standing on the rocks, waving and calling to us. His voice seemed to be getting fainter.

And then something else.

As I stared, a thick black cloud came from out of nowhere, swirling through the sky like a shoal of black fish, then spreading out, slinking like a snake, twisting, turning, up, down, circling around and around. It looked like a giant swarm of bees.

They moved as one toward me and Shona. As they did, I saw what it was: birds. Instantly they flicked and turned, back toward the castle. In a private dance for us, they weaved with perfect grace and timing around and around the castle, gliding in slow motion as though sliding down the banister of a spiral staircase, then bunching into a black ball again, spinning above the castle.

The dance went on and on as the birds whirled upward in the shape of a genie emerging from his lamp. Then, as one, they spread out and flew toward us in a fan. An enormous flock of tiny black birds pa.s.sed over our heads, chattering in a million different languages and briefly turning the sky black before they disappeared into the distance.

Seconds later, they were back, coming toward us again; more of them this time, a thick black line of them dividing the sky. They just kept coming and coming, more and more of them, to dance and swirl and break up and re-form around the castle.

"What in the ocean is that?" Shona asked eventually, her voice breathless and tight.

"The starlings!" I said.

"Starlings? Are you sure?" Shona asked.

"Positive."

"But starlings don't fly at night, do they? And certainly not out in the middle of the ocean."

I shook my head. "Look at the sky, Shona." It was brightening by the second as the moon climbed higher and higher. "This is no ordinary night."