Tom laughs like he’s been left out of a bizarre joke and someone will explain it at any moment. When I don’t say anything, he frowns at my mum then at me, his expression darkening.

‘Follow me, quickly now.’ She turns and hurries down the dimly lit passageway and I start to follow her, when Tom grabs my arm.

‘I don’t know what’s going on here, Ivy, but we should leave.’

‘If you want to go, you can. I’m sorry, but I have to stay.’

He watches my mother’s retreating back and cringes. ‘Your mum clearly needs help. Please come to the farmhouse with me. Let’s talk to the boatman and maybe he can –’

‘I told you, I’m not going.’

He clenches his jaw. ‘But I can’t leave you here on your own. Not while she’s waving a knife around.’

‘So stay.’

He glances at the door and back to me, and I shrug to show I don’t mind either way. But I do. I want him to stay, I just can’t bring myself to say it. He rubs his forehead and sighs as if he’s about to do something he knows is stupid. ‘OK. I’ll stay, but I want you to come back to the mainland with me tomorrow. Whatever’s going on here, it doesn’t feel right.’

I nod and he looks past me and raises his eyebrows. My mum has stopped at the end of the hallway and is crouched on the floor. She holds out the knife and stands up, drawing a rectangle in the air. I walk over and notice a line of white powder on the floor.

‘Salt for protection,’ she explains. ‘Step over and I will close the opening after you.’

Tom’s eyes widen and I touch his arm, willing him to go along with it. I do as she says and a sudden draught of icy air swirls about my legs. Feeling unnerved, I watch as Tom follows me, but if he notices the inexplicable change in temperature he doesn’t say anything. My mother steps over the salt and then turns and crouches down again, mumbling indistinct words as she traces an invisible door in the opposite direction.

‘The line marks the edge of protection. Whatever happens, do not cross it and never, under any circumstances, enter the lighthouse. You must do exactly as I say or I can’t promise you’ll be safe.’

I nod as if I understand, and she walks to the end of the passageway and opens a door to reveal an old-fashioned kitchen. A sturdy-looking dresser flanks the right side of the room, its shelves cluttered with glass jars and bottles. On the left are two windows and a cracked ceramic sink, and in the centre is a table with eight chairs. A range cooker sits at the far end, bunches of dried herbs hanging above it, attached to a rail suspended from the ceiling.

The slate floor and exposed stone walls make the place feel old, and the only light comes from paraffin lamps dotted about the room. Mixed with the red glow from the lighthouse, their flickering light makes everything appear strangely unreal. A fusty dampness permeates the air and there’s another odour – the bitter, pungent smell of burning herbs. Smoke billows from a brass dish on the dresser and I spot another on the floor, the pale wraiths of vapour swirling around the empty chairs like ghosts.