‘Now!’ I’m shouting. ‘Drop her now, Luc, hurry.’ His shoulders barely fit through the narrow frame, but he forces one arm and then his head out, and then somehow squeezes the other arm through the narrow space. And then he is leaning out as far as he can, holding Freya precariously at arm’s length as she flails.

‘Drop her!’ I scream.

And Luc lets go.

In the moment of falling, Freya is completely silent – mute with shock as she feels herself go.

There is the flutter of garments, and a brief flash of a round startled face – and then an almighty splash as she hits my arms and we both fall into the water.

I am scrabbling for her beneath the surface of the Reach, my fingers hooking on her face, her hair, her clutching arms … my feet slipping beneath me as the waters tug.

And then Kate is hauling me upright with Freya in my arms, and we are both choking and spluttering, and Freya’s thin scream of fury pierces the night, a choking shriek of outrage at the cold and the salt water stinging her eyes and her lungs – but her fury and pain is beautiful: she is alive, alive, alive – and that is all that matters.

I stagger to the bank, my feet sinking into the sucking mud, and Fatima snatches Freya from my arms while Thea hauls me up, my clothes dripping water and mud, and I am laughing or sobbing, I am not sure which.

‘Freya,’ I’m saying, ‘is she OK? Fatima, is she OK?’

Fatima is checking her as best she can, between Freya’s steam-engine shrieks.

‘She’s OK,’ I hear. ‘I think she’s OK. Thea, take my phone, call 999, quick.’

She hands me back my near-hysterical baby, and then turns to help Kate up the bank.

But she is not there. She is still standing in the water, beneath Luc’s window, and holding her arms up.

‘Jump!’

Luc looks at her, and at the water. For a minute I think he is about to do it, about to leap. But then he shakes his head, his expression is peaceful, resigned.

‘I’m sorry,’ he says. ‘For everything.’

And he takes a step back, a step away from the window, into the smoky depths of the room.

‘Luc!’ Kate bellows. She splashes along the shore, looking from window to window, desperately seeking the shape of Luc’s silhouette against the flames as he runs the gauntlet of the flaming corridor. But there is nothing there. He is not moving.

I picture him – curling on his bed, closing his eyes. Home at last …

‘Luc!’ Kate screams.

And then, before I realise what is happening, before any of us can stop her, she splashes through the water towards the door of the Mill, and hauls herself up.

‘Yes, the old Tide Mill,’ Thea is saying. ‘Please hurry. Fire and ambulance.’

‘Kate?’ Fatima cries. ‘Kate, what are you –’

But Kate has reached the door of the mill. She wraps her wet sleeves around her hands to protect them from the heat of the doorknob, and then she disappears inside, closing the door behind her.

Fatima darts forward, and for a second I think she is going after her. I make a grab at her wrist with my free hand, but she stops at the edge of the jetty, and we stand, all three of us, Shadow whining at Thea’s heels, barely breathing as the smoke from the Mill billows out across the Reach.

I see a shadow flash past one of the tall windows – Kate on the stairs, hunched against the heat – and then nothing – until Thea points up at the window of Luc’s old room.

‘Look!’ she says, her voice strangled with fear, and we see, against a sudden burst of flame, two figures, dark against the red-gold of the inferno.

‘Kate!’ I cry, my voice hoarse with smoke. But I know it’s no use. I know she can’t hear me. ‘Kate, please!’

And then there is a sound like an avalanche – a roaring crash that makes us all cover our ears, and cover our eyes against the blast of sparks, broken glass and burning wood that bursts from every window of the Mill.

Some vital beam in the roof has given way, and the whole thing tumbles in on itself, a bonfire collapsing under its own weight, shards of glass and flaming splinters spattering the shore as we hunch against the explosion. I feel the heat of cinders scalding my back, as I huddle over Freya in an effort to protect her.

When the noise subsides and we stand at last, the Mill is a shell, with burning beams poking like ribs into the sky. There is no roof, no floors, no staircase any more. There are only the tongues of flames, lapping from broken window frames, consuming everything.

The Mill is destroyed, utterly destroyed.

And Kate is gone.

I WAKE WITH a start, and for a long minute I have no idea where I am – the room is dimly lit and filled with the bleep of equipment and the sound of low voices and there is a smell of disinfectant and soap and smoke in my nostrils.

Then it comes back to me.

I am in hospital, on the paediatric ward. Freya is slumbering in the cot in front of me, her small fingers wrapped tightly around mine.

I rub my free hand across my eyes, raw with tears and smoke, and try to make sense of the last twelve hours. There are pictures in my head – Thea throwing herself across the narrow slip of water to try to make it to the Mill, Fatima holding her back. The huddle of police and firemen who arrived to try to deal with the blaze, and their faces when we told them there were people still inside.

The image of Freya, her chubby face smudged with ash and soot, her eyes wide and filled with the reflection of flickering flames as she watched the blaze, hypnotised by its beauty.

And, most of all, that last glimpse of Kate and Luc, silhouetted against the flames.

She went back for him.

‘Why?’ Thea kept asking hoarsely, as we waited for the ambulance, her arms wrapped tightly around a shaking, bewildered Shadow. ‘Why?’

I shook my head. But in truth, I think I know. And at last I understand Ambrose’s letter, really understand it.

It’s strange, but in the last few days and hours I have begun to realise that I never really knew Ambrose at all. I have spent so long trapped inside my fifteen-year-old self, seeing him with the uncrit-ical eyes of a child. But I am an adult myself now, approaching the age Ambrose was when we first met him, and for the first time I have been forced to consider him as an adult – equal to equal – and he seems suddenly very different: flawed, full of human faults, and wrestling with demons I never even noticed, though his struggle was written, quite literally, upon the wall.

His addictions, his drinking, his dreams and fears – I realise now, with a kind of shame, that I never even thought about them. None of us did, except for maybe Kate. We were too wrapped up in our own story to see his. I never noticed the sacrifices he had made for Kate and Luc, the career he had given up to be an art master at Salten, for her sake. I never thought about what it had taken to kick his addiction, and stay clean – I was, quite simply, not interested.

Even when his problems were shoved under our noses – that agonised conversation Thea reported to us in the cafe – we only saw them through the lens of our own concerns. We wanted to stay together, we wanted to keep using the Mill as our private refuge and playground – and so we heard his words only as far as they threatened our happiness.

The truth is, I did not know Ambrose, not really. Our lives collided for a summer, that’s all, and I loved him for what he gave me; affection, freedom, a moment’s escape from the nightmare that had become my home life. Not for who he was. I know this now. And yet, in this same moment, I think I finally understand him, and I understand what he did.

I was right, in a way. It was the letter of a man who had been poisoned by his own child, and was doing the only thing he could to spare his child the consequences. But the child wasn’t Kate. It was Luc.

We had it all backwards, that is what I have realised at last. Not just the letter, but everything. It wasn’t Kate that Ambrose was sending away. It was Luc. Why didn’t you trust him? Kate had said. But Luc had had his trust broken too many times. He thought, I suppose, that what he had always feared was coming true – that Ambrose had repented of his generosity in taking this boy into his home, loving him, caring for him. He had tested Ambrose’s love so many times – pushing him away, trying, desperately, to make sure that this person would not betray him, that this person’s love wouldn’t waver.