I’m back downstairs now, in the white hallway, and the only open door leads through to a huge kitchen-diner, done out in the same no-colour colour scheme – pale greys and sand. It’s a sort of L-shaped room, and there are double doors leading to the sitting room. The lights are off, apart from one above the cooker, which gives a yellowish glow over the whole room.

There, in the corner of the room, is a large computer screen resting on a countertop with papers and stuff littered around it: the family’s computer.

I’m tiptoeing across the floor when I hear the dog growl and, in a second, she has burst through the double doors and is bounding straight for me, a string of drool flopping from her jaws.

‘Maggie! Maggie! Oh, for heaven’s sake, that dog!’ It’s Tommy Knight’s voice but it doesn’t sound angry. Impatient and amused, I’d call it.

I’m paralysed with fear when Maggie just skitters to a halt on the white kitchen tiles, sniffing and snarling by my feet, angry and puzzled.

‘What is wrong with you, eh?’

I get a better look at Tommy Knight now that he’s not shielded by his front door. He’s tall and slightly stooped, wearing jeans high on his waist like an old man, although he’s just normal dad age. His checked shirt is tucked in and his eyes are crinkled as if he’s half smiling at a private joke. I thought the Knight twins’ dad would be twice as scary as they are, but he isn’t. In fact, only his white-blond hair gives any indication that they are related at all.

He is heading straight towards me and I edge backwards. Maggie follows, snuffling at the ground. I really think she’d attack if she could see me, but she keeps looking up and around, completely baffled at the absence of anything to accompany what she can clearly smell.

‘Come on, you – out!’ murmurs Tommy Knight, walking past me and opening the glass door to the back garden, but the dog just stays put, making threatening noises in her throat.

‘Come on. Stop that, you big silly!’

Tommy takes the dog by the collar and coaxes her away, shoving her gently out of the door and slamming it shut. The dog stops and whines at the glass barrier, and Tommy turns away, shaking his head in wry amusement.

In my fear, a little more internal gas has escaped via a silent burp.

Tommy sniffs then looks at Maggie through the glass pane. ‘You filthy beast,’ he mutters, smiling. Then he says louder, ‘Jesmond! What have you been feeding Maggie?’

He pulls out the chair in front of the computer and sits, wiggling the mouse to bring the screen to life. It’s an ancient computer: like, a truly prehistoric old Mac. I figure: This is Tommy’s computer, and only Tommy’s. There’s no way at all that Jesmond or Jarrow would do any video editing on it, and I can safely leave it alone.

He opens up an email program and starts scrolling through, opening emails, writing stuff, and it looks like he’s settling in.

Carefully, I edge away. I’m sensitive to every tiny noise I make, from my breathing to the slight sound my feet make on the tiles when I lift them up, but he doesn’t notice.

From the TV room, I hear a mobile phone’s ringtone.

I stick my head round the door. Jesmond’s laptop is next to him.

Then something happens that turns all my terror on its head.

‘Oh, hi, Mum … No, she’s gone to bed … Just, y’know, hangin’ out … When are you back? … Midnight? I’ll be in bed … yeah … Love you too. Night.’

Hearing that conversation suddenly changes Jesmond Knight from a fearsome brute into … well, a normal thirteen-year-old. ‘Love you too’? To his mum? I smile. I become less scared.

More than that, though, it’s his voice – the same as his sister’s earlier. Gone is the earthy, broad Geordie accent in favour of something much gentler. Not posh, exactly, but well on its way.

Along with Jarrow calling her father ‘Daddy’, I’m beginning to think that I’m not the only one here who’s invisible: there are parts of the Knight twins that are unseen as well.

I’m still stuck, though. For a whole hour, as it turns out, I’m trapped in the white kitchen, while Tommy Knight fiddles on his computer and Jesmond lies on the sofa in the living room, playing some game on his phone, then flicking the channels on the TV.

Eventually, Jesmond hauls himself upright on the sofa, opens the cover of his laptop and quickly types in the password. I can sort of see over his shoulder, but his fingers move too fast for me to get any idea of what he’s typing.

I’m looking so intensely, though, that I miss his father coming back towards me. For a tall guy, he moves very quietly. I just manage to squeeze aside as he comes through the double doors and his arm brushes mine.

Tommy Knight stops. He touches his forearm at the place we connected and looks around, frowning, but then carries on into the living room, where he lowers himself into a black leather armchair.

‘Did you see that thing about the missing dogs, Jesmond?’ says Tommy. ‘It’s all over the Whitley News Guardian’s website.’

It’s the way he says it, though, that makes me really pay attention. He’s looking carefully at Jesmond, as if trying to measure his son’s reaction. This is not an off-the-cuff conversation-maker.

Now, you’ve probably already made the same connection as I have, but I haven’t mentioned it yet because it was just a hunch and I wasn’t sure how relevant it was. My hunch is this: that the Knight twins are somehow connected with the missing-dogs thing. It was the encounter on the beach that day, when I was invisible and they had Lady. The other events of that day have kind of overshadowed everything, but it’s been there in the back of my mind, nibbling away like a rat chewing through an electric cable: you know, nibble, nibble, nothing, then … pow!

Jesmond is either a very good actor, or he genuinely has nothing to do with it all. His attitude is completely relaxed. He barely looks up from his laptop.

‘Hmm? Missing dogs? Haven’t seen it, Dad.’ Then he changes the subject: clever. ‘Can you run us to school tomorrow, pleeease? We’ve got luggage for the school trip.’

But Tommy is not distracted.

‘You know, Jesmond – if I do find out you’re mixed up in this, I’ll …’

Here Jesmond does look up. What threat will his dad make?

‘… I’ll be very disappointed.’

Jesmond doesn’t reply. Instead, he gets to his feet, and closes the cover of his laptop. But there’s no chance I can pull off the same trick as I did upstairs with his sister – not with his dad sitting there.

‘I’m off, Dad. Night,’ says Jesmond, and Tommy Knight’s brow wrinkles in puzzlement.

Jesmond goes through the other door, the one that leads to the hallway. He’s got his phone in his hand.

How am I going to do this? Without a proper shutdown, most laptops will turn themselves off after a few minutes. I haven’t got long to act.

I walk quickly to the back door and open it just the tiniest bit, as quietly as I can. There’s a security chain at the top, which I engage so that the door won’t open more than a few centimetres, then I start to taunt Maggie.

‘Hey, Maggie,’ I whisper. ‘You are one horrible, ugly hound – do you know that?’

That’s all it takes. Maggie starts growling, barking and whining, poking her huge muzzle through the gap between the door and the frame, trying to locate the source of the sound.

In a second, Tommy Knight is shuffling back through the kitchen, and I pass him as I head to the living room.

‘Maggie! Do be quiet, love. How did you get that door open? You want to come in, do you?’

No, please! No!

‘Sorry, love. You stay out there a bit longer. And shhh!’

Phew.

It buys me enough time to open up Jesmond’s laptop. Trouble is, the DVD drive starts up too, and the whole thing is glowing and whirring when Tommy comes back in.

Now, I can’t say for certain that Tommy Knight is spooked, but he just stops dead in the doorway, staring at the laptop. The screen is only half opened, at a forty-five-degree angle to the keyboard, giving out a glow to the room.

He stands there for maybe ten seconds – which is a long time if you count it out – then he looks back into the kitchen towards the back door, then he touches his arm again where it brushed mine, and the frown on his face gets so intense, his eyebrows practically fuse together into one.

Then he sighs, goes back to his chair and flicks through the channels until he finds something he likes: an animal documentary.

Now I’m in trouble. I can’t see the angled screen unless I get on the floor.

This I do, very cautiously. There is no way I can do all the searching that I did on Jarrow’s laptop. Not at that angle, and with all those keystrokes, each of which will make a noise.

It’s the ‘wipe’ option. Wipe everything off the laptop, restore it to factory settings. With Tommy Knight about two metres away.

He can’t actually see the screen from where he is sitting, which is a good thing, but hitting the keys silently is practically impossible. If you don’t believe me, try it sometime.

You’re probably not interested in how to wipe a computer. But the whole thing involves pressing certain keys, holding them down till the DOS appears (which is basically just a whole bunch of operating commands) and then pressing DELETE and ENTER till everything’s gone.

Which would be fine, and (just about) doable – even lying on a white carpet peering at an angled screen and hitting the keys v e r y g e n t l y with a man two metres away – except it requires the hard drive to be pretty active as it deletes stuff, and that makes a whirring noise that no amount of sound from the TV will cover, especially if the man is already spooked.

I have to press the command to delete, and I know this will set the hard drive off.

I just have to do it.

My finger hovers above it, and then I hit it. The drive whizzes to life with a loud SHHHHHH, and Tommy Knight spins round and leaps to his feet.

It’s being so close to the carpet that does it, I think. But I really couldn’t have picked a worse time to sneeze.