Emma dives beneath the surface of the pool, and I’m just about to follow her when I hear Ruby cackling with laughter. The chain has disappeared into the pool but my father is still seated on the side, his legs dangling in the water, his face red, his eyes bright with shock.

What the hell?

Ruby mustn’t have chained him up properly. But she is still laughing, doubled over with her hands on her hips. ‘Oh my God! Your faces!’ she cries. ‘That was fucking hilarious.’

I step back from the edge as Emma surfaces, treading water, and spins around to face me, a look of confusion on her face. All I know is that Dad is safe. He’s not about to drown. And then I march around the pool towards Ruby. Any pity I felt for her has now been suffocated by a white-hot fury. My heart pounds and my face burns.

‘That was priceless!’ she says, still laughing. ‘You actually though I was going to kill him. Oh, I wish I’d videoed it.’

‘You mental bitch!’ I cry.

As I approach her, Ruby’s laughter peters out. ‘Come on. He deserved it. You all did. What’s one moment of fear compared to the lifetime of crap I’ve had to put up with? I’d say we’re not even close to being even.’

As I close in on Ruby, Emma is pulling herself up out of the pool by Dad’s side. With dripping fingers, she starts working his gag loose.

‘You’re mad,’ I say to Ruby through gritted teeth. ‘You need help. Serious help.’ I can’t bear to look at her smug face. Memories of the past few weeks’ stresses and fears scroll through my mind – a catalogue of terror. How could she have put me and Emma through all that? We had no clue about my father, or about who she really is, so why did she feel it necessary to punish us? But she doesn’t seem in the least bit remorseful. She actually believes that Emma and I deserved what she put us through.

All the anxiety, terror and anger rush to the surface and I charge the final few feet towards my half-sister. With all the force I can muster, I push her into the swimming pool. She topples sideways, her hands flailing out to try and stop herself falling, but it’s too late. A shove into a warm pool might not be much of a punishment, but it feels good. The water splashes up and over me, but I don’t care about that. I have to check on Dad, see if he’s okay. I’m mad at him, too, but I’m also worried about what this ordeal has done to him. Whether he might need medical help.

‘We need some scissors or a knife to cut him loose,’ Emma says, finally pulling down his gag so it hangs around his neck. She hugs our shivering father who’s breathing really heavily, his eyes wild and staring. His ankles are tied, his hands still bound behind his back.

‘Ruby,’ he gasps.

‘I know, Dad,’ I say, stroking his arm. ‘It’s okay. She can’t hurt you now.’

‘No,’ he cries, jerking his head forward. ‘Ruby!’

Emma and I turn to see her flailing in the centre of the pool. Her hands rising up and clawing at the air while her head keeps disappearing beneath the water. When she resurfaces briefly, her eyes are panicked.

I realise she can’t swim! She’s drowning, gasping for air, and she can’t get enough into her lungs to cry for help so her struggle is silent, aside from a few pathetic splashes.

‘Get her!’ Dad cries. ‘Pull her out!’

‘We should bloody well leave her there,’ I snap.

But now she’s just a dark shape. Sinking.

‘Lizzy! Emma!’ Dad roars. ‘You pull her out of there. Right now!’ He’s struggling against his restraints, but he’s tied up tight.

Much as I want to throttle Ruby right now, I know Dad’s right. I jump into the pool, push myself down and open my eyes to see a mass of red hair floating around Ruby’s white, terrified face. Bubbles stream upwards from her mouth and nostrils. I kick over towards her and try to hook my arm around her body, but she’s thrashing too wildly and I can’t get close enough to establish a firm grip. I suddenly realise she truly is in danger of drowning. And I don’t want that. I really don’t. Despite what she did.

Emma is in the water now, and she manages to grab hold of one of Ruby’s arms. She pulls it down around her shoulders. I swim up close and yank Ruby’s other arm down around my own shoulders. Together, Emma and I kick up to the surface, hard. As we hit the air, Ruby splutters, still kicking and flailing. But Emma and I grip her tightly. Between us, we tow her to the edge and hoist her out of the water.

‘Is she all right?’ Dad asks, his eyes filled with concern as Ruby lies on her side, wheezing and retching.

Emma and I are too exhausted to reply. We pull ourselves out of the pool and sit on the side, gasping to get our breath back. My arms are aching and my legs are going to be bruised to hell where Ruby was thrashing about so much. We’re going to have to get both Dad and Ruby checked out by a doctor. But right this second, I need to just sit here. Relieved to be alive. For the nightmare to be over.