Polly trudges out of the Academy grounds at the end of another long school day, her backpack heavy with textbooks and her new shoes smeary with toad.

Polly hates that she is so bad at spells. Nobody ever wants to be in a group with her. She messes up everything. Polly pretends she doesn’t care that not a single witch at Miss Madden’s Academy of Witchcraft and Spells wants to be her friend, but secretly, deep down in that small place at the bottom of her tummy, she cares very much. Very, very much.

Lucky I have Buster, she reminds herself. Buster doesn’t mind if I am hopeless at everything. Buster likes me no matter what. Buster is Polly’s bestest, best friend in the whole wide world, which would truly be a wonderful thing if Buster were a witch or warlock.

But Buster is a monster.

Polly and Buster pretend they are not friends. After all, who has ever heard of a witch being friends with a monster? If Polly and Buster walk past each other in the street they don’t even say hello. But every day, when Polly gets home, she dumps her school bag in the kitchen, kicks off her shoes and runs straight out into the garden. She climbs to the top of the tree at the end of her yard.

Then she calls out,

This is their secret call.

Today, when Buster swings up into the tree, Polly sees he is wearing his favourite red jumpsuit. It’s a bit squeezy now that Buster has become so big and hairy, but it has lots of pockets. Buster fills his pockets with treasures for Polly to admire.

Buster tips out his pockets onto the branch. Today he has stones and sticks and three glass marbles. Then he digs his fingers even deeper and pulls out five sticky jamcakes covered in pocket fluff.

‘Ta-da!’ he says, grinning widely, his moss-green fur glowing pink with pride. ‘Afternoon tea!’

Polly smiles. ‘Thanks, Buster,’ she says, as cheerily as she can manage. She loves Buster’s jamcakes, even if they’re covered in pocket fluff, but she doesn’t feel hungry today.

Polly sits and watches her dearest friend shove one jamcake after another into his big, wet mouth.

Eventually Buster notices that Polly isn’t eating. ‘Wash da madder?’ he asks, spraying crumbs all over her. ‘Oopsh. Shorry,’ he says.

He tries to wipe the crumbs off her face with his big paws but ends up smearing her cheeks with jam. ‘I mean,’ he says, swallowing a lump of jamcake, ‘what’s the matter, Polly? You OK? You haven’t touched your afternoon tea.’

Polly stares off into the distance. ‘I messed up in spells today,’ she says sadly. ‘Again.’

Buster stops chewing. ‘Oh,’ he says, looking concerned. ‘What happened?’ He presses his finger down onto a blob of jam on his fur and sneaks it into his mouth.

Polly sighs. ‘I exploded a toad.’

‘Oh,’ Buster says again. He pauses, not quite sure what to say next. ‘Um, were you supposed to explode the toad?’

‘No!’ Polly says. ‘Of course not! I was supposed to get rid of its warts. But I accidentally exploded it instead.’

Buster watches Polly carefully. She can see that he’s wondering whether it’s OK to laugh. Polly tries to keep her mouth still but a smile twitches at the corner of her lips.

Buster lets out a hoot. ‘You exploded a toad?’ he splutters. ‘That’s hilarious!’

Polly frowns. ‘It’s not funny, Buster,’ she says. ‘Malorie Halloway laughed at me. And then I got mad and threw wart potion at her face, and Boris and Walter got some on their legs, too, and now my mother will have to pay for new school pants for them, and Miss Spinnaker made me stay behind and clean up the whole classroom. It was the worst day ever!’

‘Oh,’ says Buster. His face falls. ‘That’s bad. That’s really bad.’ He immediately shrinks in size.

When Buster feels happy he gets bigger and brighter. When he is sad he becomes small and grey. When Buster feels Polly’s feelings he almost becomes the same size as her. Polly thinks this is the sign of a true friend.

Polly leans up against Buster. He feels warm and soft and comforting. Like a favourite blanket. ‘I don’t know what’s wrong with me,’ she says in a little voice. ‘Winifred is so good at spells. I’m hopeless at everything.’

‘No, you’re not,’ Buster growls. He puts his arm around Polly and pulls her in tight. ‘You might not be good at spells and potions and all those other witchy things your show-offy big sister is good at. But you’re much better at being a friend.’

‘Thanks, Buster,’ Polly says. She allows a smile to creep across her face.

‘And you’re much better at climbing trees,’ Buster says, grinning.

‘That’s not a very useful thing to be good at,’ Polly giggles.

‘It is if you’re a monster,’ Buster says. ‘Maybe you should come to my school and learn to be a monster?’

At Darklands School for Monsters, students get to climb as much as they like. Buster gets top marks for climbing. As well as climbing, they do growling and crashing and crunching. This always sounds like fun to Polly.

‘I’d love that,’ she says, ‘but I do want to become a proper witch. A real Black Witch like Miss Spinnaker. Imagine! One that can do spells and make potions – and flies a broomstick. I don’t want to be an ordinary Green Witch like Mum, who prefers to drive a car and does her shopping at Witch Mart. My mum never does magic anymore. I mean, what’s the point of studying witchcraft and spells at school if you never use it when you grow up?’

Buster shrugs. ‘Not every witch can be a Black Witch,’ he says. ‘But you’ll be special at something, I just know it! And even if you never find that thing you are good at, you will always be special to me.’

Polly feels her heart squeeze with love for Buster. She throws her arms around his big, thick waist. ‘You are the loveliest friend a witch could ever have.’

‘Aw,’ he mumbles, glowing scarlet with happiness. ‘Thanks, Polly.’

‘And you know what else? No matter how sad or gumpy or lonely I am, you always make me feel better. Always.’

‘Oh, stop!’ Buster says. ‘Stop! Or I might explode!’

And it’s true. Buster has grown so big and full of happiness that he is almost as tight as a balloon.

‘Look at you!’ Polly giggles. ‘You look like you could take off!’

At that moment a breeze blows through the trees and Buster lifts up from the branch.

‘Buster!’ Polly grabs his paw, shrieking with laughter. ‘Think of something sad. Quick! Or you’ll float away!’

‘I can’t!’ Buster bellows, growing bigger and lighter by the second. ‘I’m too happy!’

‘Orphans!’ Polly yells. ‘Little monsters who have lost their mummies!’

She watches Buster consider this terrible thought and sees his face buckle. Immediately he shrinks and becomes heavier and darker.

‘Oh,’ he says in a deep voice. ‘That’s sad.’

And as Buster imagines for a moment what it would be like to have no mother, he shrinks, little by little, until he is almost his normal size again, and back to his usual shade of mossy green.

Polly gives him a big hug. ‘Do your friends at school know you can do that?’ she asks.

Buster looks at her, horrified. ‘I’m a monster, Polly! Imagine if the other monsters knew. I’d be teased so badly! I can never let them know how much I feel things. When I’m at school I have to concentrate hard to not feel anything at all. I can only be myself with you, Polly.’

Polly leans right into Buster’s chest and breathes in his comforting smell of leaves and wood smoke and moss. ‘It’s the same for me, Buster,’ she murmurs, her heart full to bursting with happy-sadness.

The two of them sit side by side on the long tree branch as the sky grows pink around them. Buster understands when Polly needs to be quiet. She closes her eyes and lets her thoughts drift around her like butterflies.

‘Polly?’ Buster whispers after a while.

‘Mmmm?’ Polly says, still leaning against him. She keeps her eyes closed.

‘Polly?’ Buster whispers again. A little louder this time.

‘Is it important, Buster?’ Polly says. ‘I’m kind of busy thinking.’

Buster sighs deeply. A big, growling, sort of longing sigh.

Polly opens one eye, then the other. It’s almost dark anyway. Her mum will be calling her in for dinner soon. ‘What is it?’ She smiles at her friend.

Buster smiles back wonkily and clears his throat. He shifts his bottom along the branch. ‘Um. I was wondering … I mean, I’ve just been thinking –’

Polly puts her hand on his big paw. She can see he is turning pink with shyness. ‘It’s OK, Buster. You can tell me anything, you know. You’re my best friend.’

‘Weeellll …’ he begins, turning pinker still. ‘I was just wondering. Are you going to eat that last jamcake?’