Three other people are inside the lantern room when we step back inside and one of them gives us a funny look, like, How come you’ve been out there when you’re not allowed? Or it could just be our outfits. Either way, we’re off and down the stairs.

Running back over the causeway, I see that the tide has come in quickly, filling the rock pools and lapping the lower edges of the concrete path. It’s a good job we left when we did. All the tide times are posted on notices at either end of the causeway, but still people get caught out, and kids in Whitley Bay have all grown up with scare stories of people who have risked a crossing and been swept away.

When we get to the beach, Lady bounds up to me, apparently less freaked out by my clown mask than by the total absence of a head that she saw before – and who can blame her? I can’t see the twins for the moment, but my vision is restricted by the mask – they could be right here, just behind a rock or something.

No time to worry about that now.

Lady sniffs around my feet, satisfies herself that it’s me, and rolls on her back for a tickle of her tummy, which I’m happy to provide although it feels weird through the gloves.

‘You daft thing, Lady. What happened to you?’ I’m trying to reassure her, but I’m scared that she might freak out again.

I go to grip her by her collar, but it isn’t there. Instead, I make a loop with the spare lead I’ve brought and slip it round her neck. At the same time, I peer out of the eyeholes of the clown mask to find out where the twins are, hoping of course that they have decided to walk away. I’m hoping, too, that Boydy is close by.

I’m out of luck on both counts.

The twins are standing in front of me.

Jarrow, the girl twin, speaks first, blinking hard behind her glasses.

‘Is this your dog, like? We found it. We was bringin’ it … What the hell?’

I’ve looked up from Lady, and Jarrow is staring at my clown mask.

‘Oh, it’s, erm … a charity thing,’ I say. ‘I’ve got to keep it on to raise money.’

‘Ha’d on. I know your voice,’ says Jesmond, the boy twin. ‘You’re, erm … what’s ’er name in our class, aren’t you?’

I hesitate before replying, and it’s enough space for Jarrow to chip in. ‘Yer right, Jez! It’s Pizza Face! That’s a canny mask ye’ve gorron there!’

The two of them chuckle. They both speak in broad Geordie. It’s the accent of the north-east, and it’s usually got an up-and-down musical sound that’s funny and friendly. Except sometimes, in some people, it can sound harsh and aggressive – and that’s how the twins speak, as if they’re clenching their teeth and tensing their mouths.

Jarrow turns and says something I don’t hear to her brother and they both cackle.

Now if it was me on my own, I’d have turned round and walked away with Lady. That’s the only thing you can do with people like that, in my experience. But by now, Boydy is next to us.

‘You all right, Eff?’ is all he says, but it’s enough to change the twins’ mood into something more challenging.

‘Hey! It’s Fat Lad! How are yuh, Smelliot?’ says Jesmond.

Boydy ignores the insult as if he simply hasn’t heard it. ‘All right, thanks, Jes. Just out looking for the dog. And now we’ve found her.’

‘Aye. Thanks to us. We found it. We was bringing it back, weren’t we, Jarrow?’

I interrupt. ‘Where’s her collar?’

Jarrow looks straight at me and blinks hard. ‘It didn’t have one on. We thought it was a stray.’

‘But you said you were bringing her back?’ I say. ‘Where were you bringing her back to? You were walking in the other direction!’

‘Wait on,’ says Jarrow. ‘Are wuz really having a discussion about this dog with Ronald flamin’ McDonald here? Take yer mask off and we’ll discuss this properly.’ She reaches forward to my face and I flinch away quickly.

‘No! It’s … like I say, it’s a charity thing.’

‘Well, we’re a charity too, aren’t we, Jez?’ In one swift movement, Jarrow has snatched Lady’s lead from my hand and given it to her brother, who wraps it round his fist.

Jesmond nods.

Jarrow continues, a note of true menace in her reedy voice. ‘Y’see, I don’t know for certain that this is your dog, do I? We might just have to take it to the police as a stray, and you know what they do with strays, don’t you?’

I feel a tiny bit victorious. Even though I am scared and vulnerable, I can still tell an empty threat when I hear one. I would probably have laughed in other circumstances.

‘Go ahead, then,’ I say, and I probably even sound a bit cocky. ‘She’s microchipped. You’ll probably get done for stealing a dog.’

Do the Knight twins crumble in the face of this defiance? Not a chance.

‘Microchipped?’ says Jarrow, bending down to Lady. ‘You mean, like, just about here?’ She puts her hand on the back of Lady’s neck, exactly where dogs’ microchips are implanted. ‘Just under the skin, like? I don’t think that’ll pose too much of a problem for us, do you, Jez?’

Jesmond shakes his head. ‘Last one healed up pretty quick.’

They do an about-turn and start marching away from us, pulling Lady along by her lead, leaving me gawping with a sick feeling in my stomach.

Did I understand right? Surely not.

‘Wait!’ I say.

They stop and turn, smirking.

I decide to try to appeal to their better side, if they have one. ‘Just give us the dog back. Please.’

‘Your nan’ll be dead happy to see it again, won’t she?’ says Jesmond, and I nod.

He goes on: ‘She’ll probably think a reward is in order. You know – “reward for lost dog”, like you see on lamp posts. It’s usually at least fifty quid.’

Jarrow steps closer to us. ‘We’ll take it now, eh? Save yer nan the trouble. How much have you got? Ha’way, let’s see.’

Reluctantly, I go to retrieve the ten-pound note from my jeans pocket that Gram makes me carry for emergencies. I wonder if this qualifies?

Problem: my gloved hand won’t fit into my jeans pocket. Not at first, anyway. Under normal circumstances, you’d just take your glove off to get your hand in your pocket but I cannot do that without revealing my invisibility. Awkwardly (and probably strangely, to look at anyway) I work my hand and glove into the pocket where my money is. I grab the note and then tug hard to pull my hand out.

It comes out all right – straight out of my glove, which is left trapped in my tight jeans pocket.

My apparently handless arm is left waving for a couple of seconds before I turn away. It looks exactly as if I have just pulled my hand off.

I hear a little gasp from Jarrow, and Jesmond whispers, ‘What the …?’

But in just a moment I’ve fixed it, and I’m turning back to them, offering the money with a hand that has a glove on and looks perfectly normal.

Jarrow snatches it from me. She is about to turn away when her brother stops her. He’s still gawping at my hand.

‘Did you see …? What was …?’

He just cannot put words to his thoughts, and who can blame him? What I suppose he really wants to say is: ‘Did you just see that her hand was suddenly not where it should be? Her sleeve just ended? Her glove was left in her pocket, but there was no hand?’ But he is too confused to string the words together.

Besides, Jarrow is talking.

‘Ha’d on, what about Cockney Boy here? What’ve you got? Any cash on you?’

Boydy has been very quiet. Quiet? Silent. For a big guy he seems strangely powerless when it comes to resisting these two.

‘Nothin’.’

Recovering from his confusion, Jesmond takes over what is pretty much a mugging but without any violence. ‘Nothin’ at all? You’re out and about without even the money to buy a pie? I don’t believe you. Shall I find out?’

Jesmond takes a threatening step towards Boydy, and that’s all it takes. Boydy pulls a five-pound note and some change from his pocket.

‘Thought so,’ says Jesmond. ‘And thank you both very much for the reward. Quite unnecessary of course,’ he adds, with exaggerated politeness.

Then he throws Lady’s lead down on the ground, and they both walk away in the direction they came from, back over the causeway.

But they’re behaving strangely, heads together, talking earnestly. I see Jesmond hold up his right hand in front of his sister. They’re about ten metres away when Jarrow turns back.

‘Hey, Pizza Face! I’d keep the mask on if I was you. Big improvement!’