Sam and I spend the weekend preparing for the mission and playing football (except when Mum and Dad force me to do more Walking in Heels practice). The Halloween party is straight after school on Monday. As soon as the last bell goes everyone piles into the toilets to get changed. I look longingly over at the boys going into the boys’ toilets before following Sam into the girls’ toilets. One day soon, I remind myself, I’ll be able to go through the door with the little stick man on the front. One day soon, I won’t have to panic every time I have to change for sports or a costume party. One day soon, dresses with fruit and flower patterns and sparkly animal hair clips will be washed from my memory like mud from a football kit after practice.

But for now, I’m stuck with the girly girl crowd of Melissa, Nerida and Suzy. They’ve apparently decided to keep the My Little Pony look. They still have the feathery things in their hair but now they’ve also put sheets around their middles and white shirts tied at the waist with ribbon. I have absolutely no idea what they’re supposed to be. The ghosts of My Little Ponies?

Evie is pulling on a dark blue witch costume decorated with silver stars, complete with an enormous hat and a cloak that swishes. ‘Like it?’ she asks me.

‘It’s blinding,’ I tell her.

Sam has put on her cat costume and comes up to me as she adjusts her tail. ‘Let’s hope they finish up soon,’ Sam whispers. ‘We want to get out there as quickly as we can.’

‘Yeah, I know.’ Eventually the ponies stop rearranging the feathers in their manes and the toilets are finally empty. I go into one of the cubicles, wiggle the bottom part of my costume over my clothes, pull on the pumpkin head and then press the little automatic ‘inflate’ button for the bit around my middle.

It inflates really well. Too well. In seconds, the costume has swelled to about a billion times its size. I am not just a pumpkin. I am a giant, PLANET-SIZED pumpkin.

And stuck in the toilets.

When I try to move out of the door, I get wedged in the door frame.

It turns out that you shouldn’t really press the ‘inflate’ button until you’re in a wide open space. A very wide open space.

‘Um, Josie, why didn’t you wait until you were out of the door to inflate it?’ Sam says.

My vision through the pumpkin head isn’t brilliant but it’s good enough to see that Sam is trying not to laugh.

‘Stop it,’ I tell her. ‘This is serious.’

‘Yes, yes, you’re right,’ Sam grins. ‘It is serious. Seriously funny.’ She collapses into giggles. When I keep glaring at her she takes a breath and gives one last little snort. ‘Right,’ she says. ‘Where’s the deflate button?’

‘I don’t know,’ I say. ‘But you need to find it!’ I have one more go at pushing forward but it only means that I’m jammed even more firmly in the frame of the door.

‘Okay, okay, keep your head on,’ Sam says. Then she giggles again. ‘Though that’s kind of the problem, isn’t it?’

‘Ha ha. We do have something to do you know,’ I tell her. I really don’t want to have to tell HQ that I messed up one of Mum’s most important missions because I got stuck in the toilet dressed as a giant pumpkin.

‘Okay,’ Sam says. ‘I’ve found it.’

There’s a hissing sound and the costume starts to deflate a little. As soon as it’s gone down to large continent size, instead of planet size, I push through the door.

‘Hang on.’ Sam fiddles with the button and the hissing stops. ‘Right. You’re all set. Let’s go find that skeleton.’

‘Thanks,’ I tell her. ‘You’re a life-saver.’

‘Well, more a pumpkin deflator,’ Sam grins.

In the sports hall, everyone’s gathered in their costumes. The walls have been covered in black cloth and there are – of course – little pumpkins hanging everywhere. The My Little Pony gang are already dancing to the music blaring out and most of the boys in our class are crowded around the table where the sandwiches, cakes and crisps have been laid out. I poke Sam in the arm and whisper in her ear. ‘Have you checked where Curtis is?’

Sam nods. ‘I tried to. But there’s a problem.’

‘What?’

‘He put his bag down.’ She holds up her phone with one hand and I see the flash of the tracker in the boys’ toilets. The tracker dot isn’t moving. ‘He’s probably put it in one of the lockers.’

It takes me a second to work out what she means and then I realise. We didn’t plant the tracker on Curtis, we planted it in his bag. If he’s put the bag down it means we don’t know where he is.

And that could mean trouble.

‘We’ve got to find our contact right away,’ I say.

Sam nods. ‘Let’s go.’

We start walking round the hall, slowly. I scan every dark corner, figuring our contact will be keeping in the background.

‘Wait! There he is!’ Sam tugs at my arm and turns me around.

‘Yes, I see him – let’s go.’

We make our way over to the corner of the room where a neon green-and-purple skeleton is standing by a large plate of biscuits.

Sam opens her mouth, about to give the code phrase, when I catch sight of something behind her. I yank her arm and shake my head at her, making my pumpkin head bob about.

‘Hey, Sam, it’s a bit boring over here. Let’s try over there,’ I say.

Sam immediately nods and follows me to a spot in the hall where no one else is standing. ‘What’s wrong?’

‘There’s a problem,’ I tell her. ‘Curtis.’

Sam whips round. Curtis is over by the drinks, about three feet away from the skeleton. He’s dressed as a Dalek (perfect costume choice, Curtis) – and he’s looking our way.

‘What are we going to do?’ Sam looks from our skeleton contact back to Dalek Curtis. ‘We can’t make the collection if Curtis is watching,’ Sam says. ‘Then he’ll know you’re not what you seem to be. We need to get him away from the skeleton.’

‘Okay – but how are we going to do that?’

Sam breaks into a smile as she looks at me. ‘I think your costume is going to come in handy after all.’

It turns out that there are some benefits to wearing a pumpkin costume. Especially when it’s been inflated to the max.

I move out into the middle of the hall and start to dance with Melissa and the other Little Ponies. I throw myself around to the music. People start to laugh and some of the boys join in. Noah pogos around the hall as if he’s on a space hopper. I copy him, because let’s face it, I pretty much am a space hopper. I dance closer and closer to Curtis, making sure I flail my arms as much as possible so that it looks like I’m completely out of control. Curtis edges away from me, towards the door on the opposite wall. Meanwhile, Sam is making her way over to the neon green skeleton. I see her speak and then take something from the skeleton out of the corner of my eye. When Curtis moves forward I do more bounce-and-flail dancing to keep him at a safe distance.

Which is a bit embarrassing when I realise the music has stopped.

Luckily everyone thinks I’ve been performing to make people laugh and I get a big round of applause. I do a mock bow and back away, waving to Curtis, who’s now heading towards the door with a massive scowl on his face. Being a pumpkin isn’t so bad after all.

Sam smiles as she comes up to me, pulling at her ear – our code for success.

This mission is definitely back on track.