"For - crying - out - loud," I say jerkily, staring Conor in the eye. "Accept - it. I - don't - want - another - dog."

"Last time I checked you just didn't want another Labrador," he tells me. Then his face softens. "Please, Saph. Rainbow drove all the way up to Plymouth to pick him up."

"Him?" I tap my feet, arms folded. "I don't want a boy. If I must have a puppy, I want a girl, like my Sadie."

"There weren't any girls left, and besides -" Conor drops to one knee, his eyes pleading with me. "If you don't take him Saph, I think we'll have to. Rainbow's totally in love with him. And I can't handle a pup on top of the twins."

A smile twists the corners of my lips. "Are you starting to hate fatherhood, then, Con?"

"No, it's just a little exhausting." Conor sighs, ruffles his short dark hair. I don't know why he cut it. I liked it long. "Please take the puppy. Please."

"Why don't you give him to Mum and Roger?" I demand.

"Sapphire, accept the present," Conor says icily. "For Rainbow's sake, okay?"

At that moment Rainbow herself strides in, smiling sunnily as always, cradling a small curly bundle of fluff. Two bright, dark eyes shine from a flop of black fur. The puppy struggles and Rainbow laughs, kissing the top of its head.

I look to Conor. He shakes his head helplessly.

I force a beaming smile and reach for my new burden.

It's a poodle, they tell me as I pull the pup close. A miniature poodle. I curse Conor silently. A yappy little dog. Brilliant. Just flipping brilliant.

"Does he have a name?" I ask as Rainbow chatters on excitedly about my new responsibility's fantastic pedigree.

"We've been calling him some name Conor picked. We call him Faro," says Rainbow.

I glare thunderously at Conor. You cannot be ******* serious, I mouth.

The puppy wriggles in my arms, snarling as it lunges for my hand. I whip it out of the way. Great. A puppy who likes to play with its teeth.

I flip through a list of names in my head. "Uh, I'll call him, uh, Shep," I decide.

Conor and Rainbow burst out laughing.

I hold the puppy close, protectively. "Okay, then, I'll call him Hamish. Happy now?"

I storm out, keeping the puppy tucked under my raincoat. Outside a strong Cornish squall has blown in over the cliffs; I walk briskly down the lane towards my cottage, when suddenly there's a voice behind me.

"Saph? Oh, Saph, come on, wait up."

As I turn round I see Conor running up the lane. He looks wild, dishevelled. He shakes his head at me, water droplets flying off his hair. "You've still got such a temper, Saph."

The puppy squirms against my chest, warm and soft. A delicious puppy smell of milk and salt drifts up from his dark curly fur.

"Saph, I'm sorry. It was a joke." Conor puts his arm around me. I lean into him. From the window of the cottage I can see Rainbow watching, on either side of her a green-eyed girl. The twins, Olivia and Eleanor. I smile as I look at my ten-year-old nieces. Ollie and Ellie. Already tiny versions of me, but with some of Rainbow's sweetness and Conor's humour. It's weird, but they don't look much like their mum at all, more like how I looked when I was ten. When I first discovered….

"I wanted to talk to you," Conor mumbles. "I had to tell you something. It's about the girls."

"Your girls?" I ask. He nods. Swallows.

"They are your girls, aren't they?"

"Yes, Saph!" he snaps. Then his face convulses. "But they're not Rainbow's."

"What?"

"Remember when Rainbow and I took that trip to Lapland, just before we had the girls?"

"Yeah."

"Well, I met someone there. Elvira."

"Oh my God!" My hands fly to my face. The puppy screams. I grab him just in time, hugging him for support. I kiss the top of his head, poking through the top of my jacket.

"Sorry, Hamish."

Conor sniggers again at the name.

"Shut up, Con! Tell me more. Was this why you stayed on in Lapland? It wasn't for work or whatever? Rainbow thought you were having an affair. She cried a lot while you were gone. That's why she was living with me when you came back."

My brother rakes his hands through his hair. "Oh God. I never knew that."

Images skid through my brain. Elvira, Elvira. Elvira. The beautiful, gentle, calm Mer girl whom Conor almost fell in love with when he was sixteen. The Mer girl who left him to live in the Arctic.

Let me explain.

When Conor was twelve and I was ten, our dad, Mathew, disappeared. Everyone thought he had drowned, but Con and I always believed he was alive. A year later, we both met someone very special - two children, twins, named Faro and Elvira. They were strange friends for us to have, in that they both had tails and were almost 100% Mer.

They showed us their underwater world, a world known as Ingo - a magical land filled with fish, whales, dolphins, sharks, turtles, and, most importantly, other Mer. Including our dad, who had fallen in love with Faro and Elvira's aunt Mellina and had a Mer-baby named Mordowrgi. This meant that Conor and I were half-Mer. After several wild adventures - and plenty of close calls - Elvira decided to go and live in the North, far away from our home in Cornwall, and Con and I decided to leave Ingo alone for a while. Little did we know that we'd never go back.

And now, fifteen years later, Conor's married to my best friend with two daughters, and I'm the district nurse for Senara Churchtown. Mum, who lives in Senara with us, always wanted me to be a doctor, but as I didn't want to stray too far from my past I decided to stay and be a district nurse. I don't get much work - people round here don't call for me as soon as their kids start to sniffle, like they might in other places - but it allows me to stay near my family. And the cove where I met Faro.

And now Conor's telling me his daughters are cursed with both Mer blood and a missing parent. I know from my own experience what this means: endless agony, wanting so hard to be in Ingo, to see and be with your loved one. Lucky for Ellie and Ollie that they've got Rainbow, but not knowing why they're so drawn to the sea will be so confusing for them. I stare at Conor in anguish.

His gaze meets mine. "I didn't mean to see her. I went out for a walk one night. Rainbow wasn't feeling well so she stayed in the hotel, and I went down to the seashore. She was there, waiting for me, lying on an ice floe. She's even more beautiful now, Saph. She just said my name and then we were in each other's arms."

He covers his face with his hands.

"It was the next morning that she came to find me. Rainbow was asleep and I crept out to get some air, and Elvira dragged herself onto the shore and called for me with her thoughts. That was a weird feeling, I can tell you. I went running, and then she told me. She told me she was having my baby."

"Babies," I say.

"We didn't know that at the time, Saph. Apparently it's incredibly common for Mer women to have twins."

"I know. Faro mentioned it once."

"Yeah. Well, I felt terrible about it, but I fobbed Rainbow off with some excuse and stayed until the twins were born. She dragged herself onto an iceberg a lot during her pregnancy - she got tired a lot - so the girls were born with legs. We'd agreed beforehand that if they were born with tails, she'd keep them, and if they were born with legs, I would.

"I came to her the morning they were born. She looked so pale, and they were such pretty babies."

"I remember. Mum was besotted with them."

He gives a wan smile. "Yeah. Anyway, she looked me in the eye, and she said. "You're married, aren't you, Conor?" I said yes, even though Rainbow and I were only engaged at that point, and she said, "Take them, Conor. Take our children. I know you and your wife will care for them like your own." And then she kissed the girls, and handed them to me, and then she slipped beneath the surface of the water and sank out of sight."

He wipes tears from his eyes.

"Who named them?" I ask softly, stunned. "Not Elvira…."

"No, not Elvira. Well, she gave them outlandish Mer names, but of course I couldn't possibly keep them. No, when I got back home I told Rainbow that the girls had been abandoned out on the ice, that nobody would take them and that they'd have had to go into care if I hadn't adopted them. She took to them right away, you know what she's like. She chose the name Eleanor and I chose Olivia. Their middle names are Jennifer and Matilda, after Mum and Dad, but I gave them secret middle names as well. Eleanor Matilda Hwoer and Olivia Jennifer Kerenza."

"Hwoer Kerenza! Dear sister!"

"Yes. They are sisters. Mer sisters. They should have had Mer names, but, well, I wanted them to be…. human."

Human. As Conor always wanted to be. As he always wanted me to be. As I thought I'd become.

But now, at the mention of Ingo, of Elvira, of Mer blood pure and strong, I can feel an old stirring in my chest. I look into Conor's eyes and I know he feels it too.

"That's alright, Con," I say firmly. "We'll help them. We'll help them together. We'll show them their world."

"No!" Conor's voice is a thunderclap. "They're my kids, mine and Rainbow's. Elvira's gone, and though you may hate it, Saph, Faro is too. We're normal, or almost normal, and my daughters are going to be as well."

He storms off towards the cottage.

"No, Con," I say, my voice shaking. "It doesn't matter what you say or do, they'll feel the pull. The call of Ingo will reverberate in their ears. They'll go, they'll see the Mer. We need to tell them before they're drawn in too far."

He turns, and his face is stiff with despair. He knows I'm right, that there's nothing we can do to prevent his daughters' Mer blood swimming to the surface.

"They're half-Mer, Conor," I say.

"More like three-quarters," he says gently. "Remember, Saph, we're half-Mer too."

"Ingo is strong in them," I say. Then I freeze. "I might be able to help, though. Conor, I'm going to try something. I'll be back here tomorrow at eleven, alright?"

Then I'm gone, racing through the drizzly half-light, hugging the puppy close.