Polly and Buster step into the long dark tunnel, which slopes down sharply into the belly of the earth. The ceiling drips with dank and the floor is slimy and cold. Daylight disappears quickly once they have walked a little way in, and Polly holds her hands out in front of her, edging slowly forward, hoping she doesn’t bump into anything creepy.

‘It’s dark in here,’ Buster murmurs, shuffling close behind her. ‘And spooky. Do we have to go much further? Can’t we just hide here until we know Mrs Halloway has gone? She won’t be able to see us anymore. I mean, I can hardly see you anymore! And you’re right in front of me!’

‘No, I have to keep going further,’ Polly says. ‘I feel like there’s something important the stones want me to do here. I don’t know what it is, but there’s something they want me to find.’

‘OK,’ says Buster good-naturedly. ‘But I hope the stones maybe just want you to find a restaurant or something down here.’

‘Ha, ha,’ Polly says, rolling her eyes at Buster’s silly joke.

‘What would a restaurant in a mine serve?’ Buster wonders out loud as he bumbles along behind Polly. ‘Rock cakes? Crystal shakes? Flipcakes with coal flavouredsauce? Ugh. They wouldn’t taste very nice, would they? Would they, Polly?’ He chuckles at his own jokes.

Polly ignores him. She wishes Buster would be quiet. She is finding it hard to listen out for danger over his constant chatting. It is getting darker and darker with every step, and Polly feels all her senses on high alert.

‘Um, Polly?’ Buster says eventually. ‘How are we going to find what you’re looking for if we can’t even see?’

‘I don’t know, Buster!’ she says. She stops walking, and Buster bumps into her. She feels annoyed with him even though he has done nothing wrong.

She stares into the dark tunnel ahead. It disappears into blackness. Why didn’t I bring a light? she thinks crossly. There were plenty of brinkets in the cupboard at Flora’s house. I’m so stupid not to think to bring a light.

Polly knows that without a light, they are not going to be able to go much further. She sighs. But they’ve come all this way. They can’t turn back now!

Stupid stones! she thinks. Why did they bring me here? Why couldn’t they have chosen a grown-up, at least? Miss Spinnaker would have been a much better choice.

But now Polly feels annoyed with Miss Spinnaker, too. She thought her teacher knew everything. But now she knows she’s just like all the other grown-ups who pretend they know things when they don’t.

Polly feels her heart hurting. It aches with sorrow and disappointment, and suddenly everything that had once seemed so important feels hopeless. Soon, she is so heavy with sadness she can barely stand. Her legs give way from under her and she crumples to the floor. Deep, howling sobs clench at her heart and her chest, and squeeze every little glimmer of hope out of her.

‘Polly!’ says Buster, crouching above her. ‘Polly!’ he calls again, shaking her arms and trying to pull her upright.

‘Go away, Buster!’ she yells, and her echo yells right back at them. ‘You are stupid and dumb and annoying and I wouldn’t even be here if it wasn’t for you. I should have never done that spell in the gallery to protect you. This is all your fault!’

‘Oh,’ comes Buster’s voice, small and close to her ear, twisted with hurt.

Polly hears him stand up and begin to shuffle his way back to the entrance. Part of her, a big nasty loud roaring part of her, wants him to go away. It wants to hurt him and hurt him again and again, just to see how much she can.

But then she feels a tiny flame burning deep in her belly, just the smallest, bravest flicker of hope struggling against the darkness that is filling her body like a poisonous swamp. This is the part of her that is good and kind and true.

She dips her fingers into her pocket to touch the stones, as she has so often done. They feel warm against her fingertips. But this time their warmth travels up through her hand, along her arm, across her chest and right into her heart, and suddenly her mind begins to clear.

‘Wait!’ she calls out. She sits up and sees Buster almost at the mouth of the tunnel. He is small and grey with sadness and her heart cracks to know she has done this to him.

‘Wait, Buster! It’s the mines! It’s the mines that are doing this to me. They are pulling me down into their darkness. Can’t you feel it too? I’m so sorry, Buster. I didn’t mean any of those things I just said to you. You know that. You are the best monster in the whole entire world and there is no one I love as much as you.’

Buster pauses in the entrance way and turns around. His face is pale with sorrow. ‘Do you mean it?’ he says, his voice small and broken.

‘Of course I mean it. Look!’ She holds up the pouch of stones. ‘The stones are making me feel better. And, look, Buster! They are making light, too.’

And it’s true. When she opens the pouch, the glow from the stones lights up the cave so brightly they can see every jagged detail of the rock face. Polly laughs a hiccupping kind of laugh, tears still drying on her cheeks.

‘Come on, my dearest, truest, bestest friend.’ She stands up and holds out her arms. ‘Come back, please. I can’t do this without you.’

She watches Buster’s silhouette grow bigger and taller and wider as he walks back towards her.

‘I wouldn’t have left you anyway,’ he says when he reaches her, smiling shyly. ‘I was just pretending. Friends don’t leave friends alone in spooky mines.’

Polly gives him the biggest hug ever. Then she takes his paw and they face forward again. ‘All right,’ says Polly. ‘Let’s find out why the stones want me here, OK?’

‘OK,’ Buster says. ‘Then can we go home?’

‘Then we can go home,’ Polly promises.