It was so cold in there, and so dark. And I felt so alone. Every now and then, I pa.s.sed something. Thin black fish swam in single file along the sides of the tunnel, coming from behind and overtaking me. Thick, chunky silvery-blue fish flopped by in pairs, swimming toward me and sailing over my head. Trails of seaweed hung from the walls, waving with the current and making me jump when they brushed against me.

I swam on.

The tunnel twisted and writhed about like a giant snake. Around the next corner, around the next corner, I said to myself again and again. It had to end sometime.

And then it did.

The tunnel led upward, growing lighter and warmer with every stroke until I emerged, panting and breathless, in a round pool. I took a few seconds to catch my breath as I glanced around. Where was I?

I swam all around the edges of the pool. The walls were gray rock and covered in green algae; chunks of bubbly seaweed hung down into the water like bunches of grapes on a vine.

Above the water, the pool was enclosed by walls. Gray, lumpy, dark, and cold, dripping with dampness, it was like a long-forgotten cellar, with a metal door in one corner.

I'd done it. I was in the castle. In a cellar. On my own.

What was I doing?

I shivered as I pulled myself out of the water and waited on the side, watching my tail flicker halfheartedly, flapping on the surface of the water as it faded away. My legs slowly emerged, numb and tingling. This time the numbness in my feet didn't go away. I looked down. Webbed. Even more so, like my hands.

I didn't have time to think about that, or about any of the fears I could so easily have thought about if I gave myself half a chance. Just one question remained: why was I so sure the castle offered me something? I tried to bat that question away with the rest of my dark thoughts. Whatever the castle wanted with me, I had to find out and get back to the boat. The others would be up soon.

Edging around to the doorway, I felt for a handle. A bra.s.s k.n.o.b turned slowly, creaking like an ancient floorboard as I twisted it. Despite the creaking, it turned easily enough, and I opened the door.

I inched my way up a spiral staircase, gripping a rope handrail for support. Around and around, the stairs climbed steeply and tightly. I felt as though I were climbing into the clouds, floating upward. By the time I reached the top, I was dizzy and disoriented. Another door. This time I held my breath and turned the k.n.o.b as slowly and gently as I could.

I was in a corridor, wide and long, with pictures all along the walls. Battle scenes, shipwrecks, storms at sea; the kind of thing you always see in castles like this.

I almost laughed at myself. Castles like this? How could I even think for a moment that this was like anywhere I'd ever been?

I mean, yes, from the inside it looked a little like the kind of place you might see in a book or on a doc.u.mentary or something. But there was something different about it too. Aside from the fact that it seemed to float on a mist in the middle of the ocean, something about it felt unreal a" like a film set or a cartoon. I couldn't put my finger on it, exactly, but it was just a tiny step removed from reality. As I moved along the corridor, I felt a little like an actor in a film where everything else is computer-generated animation. Unreal.

I kept glancing at the pictures to see if anything had changed while I wasn't looking a" whether the boats had moved or the storms had raged. They hadn't. Of course they hadn't. I was imagining it. I must be.

I crept on down the corridor. Ahead of me, another door lay open. I went in.

It was a smallish, box-shaped room, jam-packed from floor to ceiling with dusty books in fancy bindings, all bronze and gold. The t.i.tles were full of words I could hardly read. Most were foreign; a few were English. All looked hundreds of years old. Not exactly your light bedtime reading.

Then I noticed the window. A large rectangle that covered half of one side of the room, it was set into a recess with a small bench. I sat on the bench and looked out. The sea stretched for miles and miles, all the way to the horizon, just as it did from Fortuna. But down below, waves lapped on rocks that were gradually surfacing like bared teeth as the tide edged out. It was as though the castle stood on a podium above the rest of the world, separate from the world, floating above it as though in a dream. What was this place?

Another door took me out of the library into a smaller room. One side of the room was filled with weapons. Opposite, the wall was covered in silk banners painted with flags from all over the world. I recognized some of the shapes and colors from geography lessons back in Brightport. Others were completely unfamiliar. There was even a skull and crossbones on one flag.

I moved on quickly. The room led out to another corridor. More paintings on the walls, this time portraits. Men in naval uniforms, beautiful women smiling up at them, young men standing proud on the decks of warships, girls perched on rocks. I moved closer to examine the pictures in more detail. Hold on. Were they girls, or were they a"

What was that?

A bell clanged loudly, echoing down the corridor.

I glanced furtively around. Was it me? Had I tripped an alarm? Was someone going to come out and catch me? No! Please don't let me be captured again! Memories of being caught and imprisoned in an underwater cell after I'd awoken the kraken rushed through me with a horrible shudder. I couldn't get caught here!

There was a recess behind me, a heavy wooden door at the back of it. I jammed myself into it, my heart almost bursting out of my mouth. Pinning my body to the door, I held my breath, shut my eyes tight, and prayed for the alarm to stop.

And then it did. Stopped dead. Silence all along the long corridor. Nothing moved.

My body sagged in relief as I leaned against the door, letting out a long breath and trying to decide what to do next.

The relief didn't last long. A moment later, I heard footsteps. They were coming from behind the door, getting closer! There was no time to hide. My body froze as I stood in the recess.

And then the door opened.

I was looking into a pair of very green and very surprised eyes.

"Who are you?" asked the boy, staring back at me. He was tall, taller than me anyway, and skinny like me too. He was probably about the same age, maybe a little older, and dressed in black flared trousers and a black T-shirt. He had long jet-black hair parted perfectly in the middle and the most piercing green eyes I'd ever seen, which he continued to fix on me.

For a brief second, I remembered Millie's prediction about a tall, skinny stranger. Was this him? What had she said about him? I couldn't remember. I tended not to listen carefully to Millie's fortune-telling. For once, I wished I had.

"I a" I a"" was all I managed to say.

The boy glanced quickly down the corridor before beckoning me into the room. "You'd better come in," he said, recovering more quickly than me. His voice was silky and smooth, like his hair, and serious, like his face.

As I followed him into the room, I forced myself to speak. "I'm Emily," I said. I couldn't think of anything else to say. I looked awkwardly around me. Three of the walls were covered with maps and scrolls. There wasn't a blank inch. Every country and every ocean in the world must have been on these walls. The fourth wall had a long rectangular window that looked out to sea. Beneath it, a thick wooden bookcase held rows and rows of books, brown and bound in gold like the ones in the library. The room felt almost unreal, as though the books and maps were part of a stage set and underneath them lay a thousand years of history and mystery.

The boy noticed me looking. "They're from my ancestors," he explained.

"Your ancestors?"

"Pirates, captains, travelers of all kinds," he said. "Many ships have been wrecked on the rocks of Half Light Castle."

I nodded as though I understood.

"Look, sit down," he said, gesturing to a huge armchair. With its thick, dark wooden arms and green velvet seat, it reminded me of the furniture in the stately homes I'd seen in some of Mom's books. Mom. Just the thought of her made me ache. Where was she now? Was she trying to find me? Would I ever see her again? Each question was like a knife twisting around and around in my chest.

The boy went on staring at me as I sat down. He pulled up an identical chair and sat opposite me. "I'm Aaron," he said. He held out a skinny arm to shake my hand but almost instantly changed his mind and pulled it away.

We fell silent. I didn't have the first idea what to say. Well, come on. How many times do you think about what you'd do if you swam to a spooky castle floating on a mist in the middle of the ocean and accidentally landed in some strange boy's room?

Exactly.

He was the first to pull himself out of the shocked silence. In fact, now that I thought about it, he was more mysterious and cool than shocked. Perhaps he was used to strange things happening. Or perhaps he was just a mysterious and cool kind of boy. Either way, I was intrigued a" and thrown off balance a" by him as much as by everything else that had happened in the last couple of days.

"How did you get here?" he asked.

"Um, I swam," I said uncertainly.

His eyes opened even wider. "You swam?"

I nodded. "Through tunnels. But where am I? What kind of a place is this?"

"Half Light Castle. It's my home," said Aaron. "I don't know any other."

"You've lived here all your life?"

He nodded. "All my life. Here and nowhere else, like every generation before me, all the way back to . . ." He looked up at me through his thick black eyelashes. "No, I can't tell you that."

"Can't tell me what?"

"My family history," he replied with a grimace. "It's not exactly straightforward. You'll never believe me."

I laughed. "You think your family history is hard to believe. Wait till you hear mine!"

He didn't smile. "Trust me. It's complicated. Or it was. There's nothing too complicated now, though, as it's just Mother and me."

"Just the two of you in this whole place?"

"And a few si a"" He stopped himself, covering whatever he was about to say with a cough.

"A few what?" I asked.

"Servants," he said quickly.

"You weren't going to say that. What were you going to say?" I insisted.

Aaron shook his head and stood up. "I don't think I can tell you," he said. "I'm not sure. Look, why don't you tell me about you instead? How did you get here? It's supposed to be impossible."

"It nearly was," I said. "I tried again and again." Could I tell him about the ring? It was tight on my finger, the diamond warm against my closed palm. I could feel it almost scorching my hand, getting hotter. What was it saying? Tell him? Or keep it to myself?

Why should I keep it secret, anyway? I had nothing to hide. "Look, if I tell you, you promise you'll believe me?" I wanted to tell him. I felt I could trust him. I don't know why. There was just something about him that I could connect with. As though we spoke the same language.

"Why would I do otherwise? Why would you lie?"

"OK," I said. "Well, it was this. It kind of led me here." I held my hand out and opened my palm to reveal the ring. "Now, I know you'll think I'm making it up or you'll think I'm crazy or something, but I promise I'm telling you the a""

"Where did you get that?" Aaron reached out and grabbed my hand, pulling it toward him to look closer. His voice shook so much I could barely understand what he'd said. He swallowed hard, catching his breath. His pale face had turned even paler. "Where did you get it?" he repeated.

"I a" I found it," I said uncertainly.

"Do you know what it is?" he asked.

"Well, I a" yes, I think I do." Did he know what the ring was? Had he heard of Neptune, heard the story?

"I've never seen it," he said in a whisper. "Not the real one!"

He fell silent, squeezing his mouth into a tight line and his eyes into slits while he thought. "All right," he said, making up his mind about something. "We've got time. Come with me."

With that, he motioned to me to follow him to the door. Glancing down the corridor again, he nodded back to me. "Come on," he said. "I want to show you something."

Aaron led me down a maze of corridors, scurrying quickly along till we came to a thick wooden door with bars and bolts across it. I followed him outside. Below us, the sea washed against rocks in the semidarkness. We ran around to the front of the castle and back inside through a small arched door. Following Aaron inside, I felt as though I were stepping further and further into a dream. Was any of this real? I mean, it felt real. The bricks of the castle were thick and hard, the rocks below were jagged and cold. But, still, something in the at mosphere made me feel as though I were floating, suspended just above reality, as if the castle really were floating on the mist.

I closed the door behind me.

We were in what looked like a small church. A tiny chapel in a remote wing of the castle. A few rows of seats all faced a raised platform at the front. Stained-gla.s.s windows were filled with pictures of biblical scenes.

I followed Aaron to the raised platform. Right at the back of it there was a chest. He opened it. "Look," he said, pointing inside.

I peered into it. It contained a gla.s.s cabinet and, inside that a" two rings. I looked closer at the one on the left, comparing it with the ring on my finger. It was identical!

"But that's a" but they're a""

"Imitations," he said. "My great-grandfather made them. From the descriptions, from the stories pa.s.sed down through generation after generation."

"What stories? What descriptions?" I asked, my head spinning. "Do you mean about Neptune and Aurora?"

"You know?" He gasped. "You know the story?"

"That's all I know," I said. "Please, tell me."

Aaron moved away from the cabinet. "When Neptune and Aurora married, they cast a spell on their rings. While they were held by a human and merperson who were in love a""

He glanced at me to check that I understood what he meant, to check that we were talking about the same thing. Maybe to check that I didn't think he was ridiculous for believing in mermaids. I don't only believe in them, I thought, I am one! But I wasn't going to say that. Not yet. Not if it was just a story. Surely boys like him didn't really believe in mermaids. Not that I'd ever met a boy quite like him. I don't know if I'd met anyone like him before. All I knew was, I wanted to hear all about whatever he had to say.

I nodded for him to continue.

"As long as the rings were worn by one from land and one from the sea who loved each other, there would always be harmony between the two worlds," he went on. "And there was. For the brief time the marriage lasted, there really was peace between land and sea. No ships were wrecked on rocks; no cargo was stolen; no sirens lured fishermen to their watery graves. Just peace. The two worlds thrived together. It was a magical time."

"And then she left him," I said, remembering what Shona had said about her history lesson.

Aaron's green eyes bore down on me. "She what?" he asked angrily.

"She a" she left him?" I said more uncertainly. "Didn't she?"

"You know nothing!" he snapped. "Believing in such nonsense. How dare you?"

I pulled at my hair, twisting it around my fingers. "I'm sorry," I said. "I thought she did. I thought she broke his heart. I'm sorry."

"She did not leave him," Aaron said firmly. "She loved him more than anything in the world. I'll tell you what her love for him drove her to do."

I clamped my mouth shut. No more interruptions.

"She loved him so much and believed so strongly in the magic they had created that she attempted the impossible. One night, she decided to show him what she could do out of love for him. You know what she did?"

I shook my head.

"She thought she could swim underwater to his palace. She believed their love was so great, it surpa.s.sed the normal laws of her human world. She believed she could become a mermaid. She drowned."

Neither of us spoke for a long time. As we stood in the silence, it felt as though the chapel were the whole world. As though the sea outside the window were there only for us. We were somehow at the center of everything, the center of something so important that a" that what? I couldn't tell.

"It was her birthday. She'd wanted to surprise him as her present to him," Aaron went on. "Her own birthday, and she wanted to surprise him. They'd only been married two years and a week."

"Go on," I said softly.