At that moment, Finn looks around. 'Where's Sacha?'

'She'll be here in a minute,' says Dad quickly, with a glance at me. 'Shall we go and make Bleater a bottle?'

Something is very wrong. I stand bewildered, holding Finn's little backpack. Bianka takes my arm and leads me out of Kit's earshot. 'She's in the hut,' she whispers tersely.

I stare. 'Tell me she hasn't got hold of anything.'

'I'm so sorry! Look, I shadowed her all day, every day, until she was ready to punch me. But yesterday lunchtime I had to go and help a group of Year Nines with their history project . . . I got volunteered.' Bianka's mouth twists in regret.

'Yes?' I'm impatient. 'Go on.'

'She must've borrowed a phone, texted a dealer and met him outside school. That's all I can think of. I noticed her coming back, and she said she'd just nipped out to the dairy for a chocolate bar. But I could tell from the way she looked at me . . . you know?'

'Oh, yes. I know.' I can easily picture the new, deceitful Sacha. I know that creature all too well.

'She took some last night. I don't know how, I don't know when, but she was manic, up and down all night. I didn't want to spoil Finn's homecoming so I thought I'd just keep an eye on her, which was a stupid mistake because as soon as you'd gone this morning, she disappeared. It took me a while to find her in the smoko hut.'

'Okay.' Rage and sorrow and fear have been seething in me for months, and now they overflow. I know what I'm going to do. I begin to back away, ready to run. 'Have you spoken to her?'

Bianka shakes her head miserably. 'She wouldn't even open the door to your gorgeous dad. She went psycho. She-'

I don't need to hear any more. I sprint past the kitchen door and along the path to the hut. Its windows have been covered again, and the door is locked. I hammer on the flaky wood. Shards of paint fall away. 'Sacha!' I bellow.

There's clumsy movement inside, and the sound of something clattering; then a muttered 'get f.u.c.ked'. It isn't a girl speaking. It's the voice of that horrible Sacha beast.

Resolve settles in me like icy water. It weighs me down and makes me cold. 'I won't get f.u.c.ked,' I say, very clearly. 'It's time for you to listen. So listen carefully. Are you listening?'

A snarled obscenity.

'Do you know what really happened to Finn?'

Silence.

I lean my face close to the door, my lips almost touching it. 'I'll tell you. You thought creatures were coming to get you. Remember that, Sacha? They were creeping along the balcony and into your bedroom. You were scared out of your wits.'

'Shut up!' The voice is coa.r.s.e. Ugly. 'Shut the f.u.c.k up.'

'All night long they were whispering. You saw their gleaming eyes, a face at the window, crawling things. And in the morning you weren't in bed, were you? No. You were sitting in your cupboard. How do you think you got there?'

I feel a powerful thud just in front of my face, followed by the smashing of gla.s.s. A bottle, I'm sure, hurled at the door. Then another. The old timber shivers at each impact.

I raise my voice. 'During the night, you heard something on the balcony. You ran out there. Remember, Sacha? Remember unlocking your door and running outside? Yes. I think you do.'

Another smash. The door buckles slightly.

'You caught a creature prowling around behind your door. You actually caught one! So what did you do? You hurled it over the rail.' I'm choking on my rage. It fills my chest. I have to breathe hard before my next words. 'That was Finn.'

I want her to taste my horror. I feel as though I'm slicing into this imposter with a sharp knife; I have to cut the real Sacha out of her. She must have run out of bottles to throw, because this time it sounds more like a kick. 'He's got no spleen,' I shriek, twisting the knife. 'He's got a broken arm and they had to dig two pieces of skull out of his brain. G.o.d knows what the future holds for him. And you did this to him!'

Volcanic pressure is building behind my forehead. I boot the door myself, kick it with all my fury. I kick five, ten times until it splinters and caves, and my foot goes through.

I hear the bolt drawing back. Sacha is standing in the doorway, her eyes black, her mouth open in a soundless scream.

'It's not true,' she wails. 'It's not true.'

'Oh yes, it is. I saw you do it. Shall I go and tell him? Shall I tell Finn how much his sister loves him?'

I see it in her eyes. The knowledge. The shock. Then she staggers back into the hut, pressing both arms over her face.

'Where is it?' I tear down the curtains and begin to ransack the room. I'm seriously considering whether to scatter petrol and set a match to this lair. I gather up a home-made pipe and lighters and tape and all the other bits of paraphernalia, and throw everything into her bin. 'I said, where is it?'

'I got it on credit. Just a point. It's all gone.'

'Where is it?'

She pulls one of the tiny bags from inside her bra. A few crystals; innocent, like rock salt. 'I shouldn't be on this earth,' she weeps, as I wash them down the plughole.

'No,' I say bitterly. 'You probably shouldn't.'

I hear footsteps outside. Dad stops at the door of the hut, taking in the scene in an instant. 'Finn's flaked out. He's asking for you, Martha.' He jerks his head back towards the house. 'Go on. Go and make him comfy, poor little man. Maybe I can help here.'

When I leave them, my father is sitting on the floor beside Sacha. He's doing what I haven't been able to do; what I think I may never again be able to do. He's put his arms around his granddaughter, and is telling her she is still loved.

When I look out of the kitchen window ten minutes later, Dad's car has gone. At lunchtime Finn asks for him and Sacha. I say they've gone for a walk, and Kit stares at me. After all, this is Finn's big homecoming day. Charlie and Dad spent yesterday evening making a cake and banners; the idea of anyone swanning off for a stroll is bound to raise an eyebrow.

After lunch, Finn falls asleep on the sofa. Charlie glues himself to his twin, warm and contented, squashed up close. Bianka sits nearby, reading a book. I know my time is running out. Well, let Kit tackle me. I've come to a decision.

He's loading the dishwasher when he finally asks the dreaded question. 'She's relapsed again, hasn't she?' he says quietly. When I nod, he slams the dishwasher door shut with his foot. 'f.u.c.k.'

'Kit.'

'Stupid girl,' he growls. 'Okay, okay. We've got to get help.'

'Kit.' I feel a cold sweat on my forehead, as though I'm about to be sick. I'm quivering at the top of a high diving board, gazing down, down, knowing I have to jump. This could be the end of everything.

'Get on to a counsellor,' he's saying. 'Tell them all about-'

I have to shout over him. 'Kit! Please listen.'

He stops talking. I shut my eyes for a second, and then I hurl myself off the board.

'She was paranoid,' I say. 'She thought there were evil beings stalking her. I think maybe it started months ago, the paranoia, but it got worse and worse. Sometimes she heard them whispering, even caught glimpses of them. They terrified her. Then one night, the night you came home from Dublin, she actually caught one lurking outside her bedroom door.'

Kit has turned to stone, the colour rapidly leaving his face.

'She knew it was one of her tormentors. She picked it up and she . . . she . . .'

'No. Jesus, Martha. No.'

I'm struggling to utter the terrible words. 'She threw him. She threw him off the balcony.' I bury my eyes in the palm of one hand, feeling tears run through my fingers. 'I wasn't quick enough.'

'You saw this happen?'

I step close to him, supplicating, trying to put my arms around his neck. 'I thought if I could only keep it to myself-'

'You thought you'd feed me a pack of lies! You thought you and I could live together for the next fifty years, whatever, and you'd be lying every second of it.'

'I didn't know whether you could forgive her.'

'Didn't you? No.' He throws my arms away from his neck and strides out of the room. A minute later, I hear a distant thud as the studio door slams.

I spend the next hour in a blank daze, lying on the floor by the stove with my hand on m.u.f.fin's warm back. Kit doesn't reappear. I imagine him hunched over a bottle, raging to himself. I don't blame him. His world has finally collapsed. His stepdaughter tried to kill his son, and his wife covered up for her. He must be wondering if there is anyone he can trust. I fear for us all.

The shadows are long on our lawn when I get a call from Dad. The line is faint.

'She's told me,' he says simply.

'Can you look her in the eye?'

'Who's going to cast the first stone, Martha? Not me, that's for sure.'

I sigh. 'I think Kit might.'

The line crackles. 'Look, come down to the beach. We're here. We need to talk to you.'

There's no sign of life from the studio, but Bianka says she'll keep an eye on the twins. They're both asleep now, piled in a soft heap on the sofa.

I find Dad's car near the school, in the spot where we parked the day we first came to Torutaniwha. It's a golden evening, just as that one was. Tussock gra.s.s shivers in a light breeze. I wander through the dunes, reliving the magic of that day. It's all over now. I wish I could wind back time and start again.

The beach is empty except for two familiar figures who sit close together among the rocks at the foot of Hinemoana's hill: a gaunt girl with bony arms, whose spray of curls seems to flow on the salt breeze; and a wiry, grey-haired man whose eyes miss nothing. I love them both. I approach along the foresh.o.r.e, my feet sinking with every step, the wind freshening around my ears. As I come closer I see that they're sharing a rock. They seem to be watching a party of gannets, soaring and diving beyond the breakers. Sacha has her head on Dad's shoulder, and his arm is around her.

He watches my approach with a tranquil smile. 'We came to ask her advice.'

I glance around, but we're alone. 'Ask who?'

'Hinemoana,' says Sacha jerkily, without lifting her head from Dad's shoulder. 'We thought she'd understand.'

I choose my own rock to sit on. It's covered in seaweed and little sh.e.l.ls. Nearby, a hunting gannet plunges into the waves and reappears with a struggling fish in its beak.

Dad glances at the cliffs. 'Sacha thought of Hinemoana because she knows what it's like to be enchanted by evil. And she found her own way out.'

'We'll have to go back to England,' I say.

'And the boys?' persists Dad. 'And Kit? This is their true home. I've seen that for myself.'

'Kit doesn't have any choice. It's my job that got us the visas.'

'I don't want that!' Sacha must be coming down from her last smoke, whenever that was. She looks drained and edgy, but determined. 'I've done enough damage without making all of you leave Torutaniwha. It's the last thing I want. Look, Mum, I have to live with this for the rest of my life. I have to find a way to go on, knowing what I did to Finn. And just saying sorry is never, ever going to be enough.'

The temperature seems to drop abruptly, as the sun leaves the beach.

'The three of us have come up with a plan,' says Dad. 'Hinemoana had her say, too. At first Sacha felt that she didn't deserve to live, after what she did. She felt that there was only one way out.' He smiles at his granddaughter. 'But she will not drown herself here at Hinemoana's hill, no matter how poetic that might sound. She knows that she has a future- don't you, Sacha? She knows that her family want her to be made whole and well. She also knows that she can defeat this evil if she fights it with everything she's got, even if that means leaving the people she loves.'

'We can defeat it,' I insist desperately. 'We'll go to a doctor. We'll find a rehab place of some sort.'

Dad holds up a hand. 'Please listen, Martha. Listen to our plan.'

'I don't want to hear it!' I wail, pressing my hands to my ears.

'Mum.' Sacha slides from her rock and kneels in the sand in front of me. 'Just saying sorry isn't enough for me, this time. It can never be enough.'

So I listen, as the evening light softens Hinemoana's limestone face. I listen, and I argue. In the end, I can't fault the logic of their plan. It makes perfect sense. It gives us all a way out. But it's too much to bear.

Venus has risen as we begin to make our way back up the beach. The three of us don't speak much. We've made our decision. Even the waves are subdued.

'Look,' says Sacha, stopping to squint into the gloom. 'Someone's coming to meet us.'

Peering down the pale curve of the surf line, I make out a human shadow, a streak in the gunmetal twilight. He lopes steadily closer, hands in his pockets; no hint at all of a drunken stagger.

'It's Kit!' cries Sacha, and I hear her feet sinking in the wet sand as she runs. I'm about to follow when I feel a firm hand on my arm.

'They've a lot to talk about,' says Dad.

'She might need help! He's probably plastered.'

'He looks sober enough to me. Have a little faith, Martha.'

I watch Sacha's sprinting figure. 'Will you tell her about her father?' I ask.

'It's not my secret to tell.'

'But you'll be tempted.'

'Kit is her father.' Dad nods up the beach. 'Look.'

Ahead of us, the two silhouettes merge.

Forty-two.