When Bianca woke up, she found she was lying on top of her duvet, on her bed, still wearing her coat and wellington boots. Her first thought was of Finn. She hoped wildly that last night had been a bad dream, but her clothing told her it hadn’t been. Her heart felt heavy. A storm of tears was brewing behind her eyes. Poor Finn, all alone in the park. Was he cold? Was he frightened?

‘Being sad won’t help him,’ she scolded herself. She had seen how it had disarmed her dad, who usually had a plan for every situation, and her mum couldn’t stop crying once she had started. Bianca decided she would be brave and strong for her brother.

Shivering as she got up, she breathed out a white mist. Her bedroom was freezing. Why hadn’t the heating come on?

Three of the walls in Bianca’s bedroom were painted lilac-grey but the fourth wall was her favourite. It was papered with a blue-and-white winter woodland scene. Reindeer peeped round trees, foxes were curled up in underground dens, and squirrels perched on snowy branches, clutching acorns. She faced the woodland, doing star jumps until she felt warmth returning to her numb limbs. Running on the spot as fast as she could, she chanted under her breath, ‘Wake up, wake up, wake up!’

She needed to switch on her brain. She’d made a promise to free Finn from the ice and find the person responsible. She intended to keep it.

Perhaps, if it’s sunny today, the ice around Finn will melt, she thought, going to the window and whipping the curtain aside.

The view startled her.

Yesterday it had been warm enough for her not to need a jumper under her coat. Today thick impenetrable clouds hovered over the city rooftops like a flock of sullen sheep waiting to be shorn of their snowy fleeces.

On the outside of the window, frost fractals had created patterns that looked like tiny ferns and ostrich-feather fans.

Bianca pulled her coat around her and wondered if this was what the grown-ups called a ‘cold snap’. It hadn’t been like this yesterday.

Today was Saturday. No school meant she had the whole day to investigate. She would start right away. Throwing open her wardrobe, she pulled out her warmest clothes, putting on thermal underwear, thick leggings and two pairs of socks. A thought made her open her bottom drawer. Inside was a jumper she’d never worn. Her mother had knitted it for her as a Christmas gift. One arm was longer than the other, the neck gaped, and it hung down almost to her knees. The wool was a soft warm purple and it had a white B on the front, for Bianca. She’d tried to hide her disappointment when she’d opened it last Christmas. Her mum had declared that she looked wonderful in it, but Bianca had hidden it in her bottom drawer and not looked at it since. It might cheer Mum up to see her wearing it today. She put it on over her thermal vest and looked at herself in the mirror, nodding. This felt like the right uniform for a mission to save Finn.

Pulling her coat back on, Bianca went to her desk, picking up her pocket-sized orange diary and a pen. She would need a notebook, to write things down, like the police officer had last night.

She marched along the landing. The low rumble of adult voices drifted up the stairs and she wondered if her parents had gone to bed at all last night. She didn’t think they had.

The first thing to do was to inspect the scene of the crime. Something had happened last night in her brother’s bedroom that had led to him leaving the house and ending up in the park, frozen on a pedestal of ice.

She opened her brother’s bedroom door. A higgledy-piggledy fantasy land sprawled across Finn’s carpet. Toy train tracks wove around exotic animals, a pirate ship, a teddy bears’ picnic, a giant plastic Tyrannosaurus rex and buildings constructed from brightly coloured blocks. Bianca tiptoed across the floor, not wanting to disturb anything. She studied the room in sections, hunting for anything out of place or out of the ordinary. She checked the window. It was closed, locked and patterned with frost. There were no signs of it having been opened.

Finn’s bedding was ruckled, and Bianca’s heart twisted as her eyes landed on his favourite cuddly toy. Sposh, a white rabbit wearing a cheerful red scarf, lay abandoned on Finn’s pillow. She picked up the bunny and hugged it. Finn had been three when they’d first visited the city zoo together. He’d fallen in love with the bunny in the gift shop. He’d taken it from the shelf, chattering away to it as if it had been sitting there waiting for him. Her parents had had no choice but to buy it. Finn and Sposh had been inseparable ever since. Except now Sposh was here, and Finn was in the ice. If Finn had left the house of his own will, in the dark, Bianca knew he would have taken Sposh with him.

Perching on the edge of her brother’s bed, cuddling his bunny, Bianca carefully went through the events of the previous day. It had been a Friday. They’d gone to school, like normal. Dad had picked them up and taken them to the library, like normal. They’d come home and eaten dinner: fish fingers, chips and peas. Then she’d asked to see Finn’s library book. It had a sparkling silver cover and she had wanted to look at it, but he wouldn’t let her. They’d fought. Mum, exasperated by their bickering, had sent them to their separate bedrooms. And then they’d gone to bed, like normal. But something desperately abnormal had to have happened for Finn to leave the house in the night.

‘There’s something I’m not seeing!’ she said to herself.

A mournful voice inside her head told her that the thing she wasn’t seeing was her little brother, sitting cross-legged on his bedroom floor, playing happily with his toys, as he always did on a Saturday morning.

There was a pile of books on the floor in front of the bedside table, and Bianca thought she might look at Finn’s library book. The one he hadn’t let her see yesterday. She leaned her hand on the pile, to bend down, and found that the top book was wet, as if a glass of water had been spilt on it. She wiped her hand on the bedcovers, and read the stack of spines. None of them sparkled like his library book had. Thinking that Finn might have been reading it when he fell asleep, Bianca checked down the gap where the bed frame butted up against the wall, then underneath the bed. No book. Picking her way across the toy-strewn floor, she looked in Finn’s bookcase, but it wasn’t there either.

‘It must be here somewhere,’ Bianca muttered to herself, puzzled. Eventually she gave up, assuming the book must be in Mum and Dad’s room, or downstairs. She pushed it to the back of her mind. She was determined to complete her mission and conduct a thorough search of the house for clues.