Bianca pushed her imagination to describe every vivid detail of her story, and, as she saw a pair of brown hands reach through the door, she dropped her orange book, grabbed them and hurled herself forward, back into Snow Haven.

‘Watch it!’ Casper cried out as Bianca collided with him, knocking him off his feet. The pair of them landed on a carpet of snowdrops.

‘It worked!’ Bianca exclaimed, sitting up and looking about in astonishment. ‘I’m back!’

‘Hold still,’ Casper said, gently lifting an icy ladybird with blue spots from her cheek. He held it up so she could see it. The ladybird flicked open its spotted elytra and launched itself into the air.

Bianca gave him a spontaneous hug. ‘Thank you for coming to get me.’

‘’S all right,’ Casper muttered, suddenly bashful. ‘What’re friends for?’

‘Oh, Pordis!’ Bianca flung her arms round the reindeer. ‘I’ve missed you.’

‘And I you.’ The reindeer nuzzled Bianca’s neck.

‘How long have I been gone?’

‘Moments have passed.’

‘But how many minutes?’

‘What is minutes?’

‘About twenty,’ replied Casper. ‘Maybe half an hour.’

‘We don’t have much time. We need to find Jack.’

‘Is Jack the tall, thin one? Wears a top hat?’

‘Yes.’

‘Jack’s gone. They all have. They got into a big sleigh pulled by a unicorn. And, Bianca . . .’ Casper swallowed. ‘They’ve got Finn.’

‘Yes, I know.’ Bianca nodded.

Casper was surprised by her calm reaction to this news.

‘Did you see which way they went?’ Bianca asked.

‘It looked like they were heading to Winterton.’

‘Then that’s where we need to go,’ Bianca said, hurrying out of the frost garden. ‘We need to reach them as quickly as we can.’

‘Monodon will take us,’ Casper said, picking up his pace as Bianca started running.

‘When you fell, did the slide take you down to Monodon?’ Bianca asked as they sprinted down the passageway.

‘Yes, but—’

‘Great.’ As she approached the hole, Bianca dived down it head first. ‘Meet you by the ice raft, Pordis,’ she called out as she zoomed away down the slide.

Bianca whooped as she shot out of the slide, bringing her arms and legs together into a dive. She heard Casper’s whoop of delight as he rocketed out of the chute above her. He hit the water seconds after she did.

Monodon was waiting. The whale swam to Bianca so she could grab on to his horn, before collecting Casper. When they arrived at the ice raft, Pordis was climbing aboard. Bianca pulled herself out of the water, marvelling that she didn’t feel the cold here. Monodon moved around to the shore side and pushed their Arctic craft back towards the Winterton cove.

‘Casper, look.’ Bianca pointed. Snow Haven was bathed in a delicate lilac-pink wash of colour. ‘The sun is setting.’

‘It’s not done that before.’ Casper looked at her. ‘Does it mean something?’

‘It means time is running out. The winter solstice is starting.’

Casper closed his eyes to speak to the narwhal, and Monodon’s tail beat the water faster, propelling them forward at a ferocious speed.

As their sheet of ice approached the beach, Bianca, Casper and Pordis jumped ashore. Bianca climbed aboard her reindeer’s back, then held out her hand to Casper. ‘Pordis says she will carry you.’

The stoic reindeer cantered up the rocky path with the two children on her back.

‘Where are we going?’ Casper asked as they passed under the sign for Winterton.

‘To find Jack,’ Bianca replied. ‘The sleigh you described should be easy to spot.’

There were a few children on the rides, but most of them were moving away from the fairground towards the snowball field, and Bianca could hear music being played on whistles. She and Casper slid off Pordis’s back and they joined the crowd, moving past a brightly painted open-sided wagon.

Inside the wagon, Quilo was playing the keyboard of a calliope, a steam organ. He looked delighted with himself as he played his merry tune, calling all the children to their next and final entertainment.

The snowball field had been transformed from a battleground into an amphitheatre. Rising tiers of snow seats fanned out around the stage, an enormous circular disc of ice. A chain of cheerful pink lanterns was suspended between tall posts encircling the arena. Children and their winter creatures were flooding into the theatre with mugs of hot chocolate and punnets of frozen fruit. The seats were filling up fast. Behind the circular stage was a blazing backdrop saying SNOW CIRCUS, and Bianca saw a trapeze above the stage.

This is where they’re going to collect the frozen hearts for Ishild, she thought, searching the crowd for Jack’s tall, thin figure.

‘The sleigh is behind the stage.’ Casper pointed. ‘I can see the back of it.’

‘That must be where Jack is,’ Bianca said.

Casper moved to go with her, but Bianca stopped him. ‘Casper, I . . . I need to talk to Jack alone.’

‘But what if you get trapped again? Jack is dangerous and they’ve got your brother.’

‘If I need help, Pordis will come and get you.’ Bianca tried to sound as if she knew what she was doing.

Casper studied her grave face and frowned. ‘This isn’t just about your brother, is it?’

‘No.’ Bianca felt a flash of guilt. Casper had no idea that his heart was in jeopardy, that there was another world in which he had a dad who loved him dearly and wanted his son back. ‘It’s bigger than that. It’s bigger than all of us. It’s too big to explain.’ The music from the calliope wagon was getting louder as two enormous polar bears dragged it onto the stage. Quilo was standing up on his seat as he played, acting like a rock star. ‘I don’t have time to tell you the whole story. The show is starting.’

‘You go,’ Casper said, stepping back. ‘I’ll get a seat as close to the front as I can. In case you need me.’

Bianca shot him a grateful smile and ran down the field, heading for the backdrop behind the circular stage. There were no guards. She guessed that, with her safely gone from Winterton, Jack and the others thought they didn’t need them. All the other children were in thrall to winter.

Slipping behind the backdrop, a woven tapestry of snowflakes, Bianca found herself in a backstage area. Four tall, thin white tents were pitched in a semicircle, their doorways decorated with triangular flags of pale smoke, fluttering without a breeze. The sleigh, with the unicorn still tethered to it, was parked in front of them.

Bianca felt drawn to the unicorn. How magical would it feel to touch its silver mane and look deeply into its eyes? But she was on an urgent quest. She turned her attention to the four tents. Peering in the first, she saw it was empty, and guessed it was Quilo’s as he was on the stage. A noise startled her and, quick as a flash, she slipped inside, peeping through the gap in the fabric. She saw Jack, wearing a white ringmaster’s jacket with tails, a white top hat and carrying a silver cane, stride past. After a moment’s hesitation, she came out of hiding, opening her mouth to speak, only to see Jack leap impossibly high, rising above the tapestry backdrop. Reaching out a snowflake hand, his fingertips shot out ice, making a path in the air. Holding the silver cane high, Jack Frost slid down the path as it was created. ‘Children! Creatures of winter! Welcome to the Snow Circus!’

The audience cheered, clapped, honked and howled.

Running to the backdrop, Bianca looked under it and saw Jack descend onto the circular stage where Quilo was playing a wild tune on the steam organ.

‘Let us begin,’ Jack cried, ‘with the winter animal parade!’

And all the snow creatures, whom the children loved so dearly, filed down to the stage to march around in time to Quilo’s mad music, while the children whooped and applauded them.

The show had begun, and Bianca didn’t know how to stop it.