On this particular sultry summer's afternoon, Anna found herself sprawled face-down on the floor of her bedroom. Having heard a clattering noise from behind the bookcase in the corner, she was now pulling out piles of books and paper from the gap between the floor and the bookcase to investigate. Some of these books had sat there for years, and she sneezed as the dislodged dust drifted into her nose. This was not how she had planned to spend her afternoon off, for the sake of rescuing some pencils or jewellery or toys that had fallen off the top of the bookcase. Or maybe she was about to make acquaintance with some rodent roommates that had previously gone unnoticed?

Anna stretched out her hand to feel for any interesting debris. There were hair ribbons and pins, an old glove that was far too small for her now, and some little hard round things that, when pulled out into the light, turned out to be beads from a broken bracelet rather than something more sinister. She got up onto her hands and knees and tipped her head upside down to look for anything she had missed, half-hoping and half-dreading to see a mouse hole. But instead of a continuation of the white wall panelling, the wall behind the bookcase looked very much like...

A door?

Without a moment's hesitation, she flung open the bookcase's glass doors and drawers and threw all of its contents onto the floor. She spent far too much time outdoors on solitary picnics, stealthy kitchen raids and tree-climbing adventures to care much for these ornamental books which she probably should have read by now. By the time she had finished, her room looked even more like an explosion than usual. Anna hoped her mother wouldn't choose this time to pay her a visit and witness the sight.

Careful not to topple it and crush herself, Anna heaved the bookcase away from the wall. She was faced with a plain white door, quite ordinary-looking, though its door handle had been removed, allowing the bookcase to stand close enough to the wall to conceal it. Anna frowned. How could she not have known about this door? And this bookcase has always stood here, hadn't it? Eagerly, she pressed her face to the gap between the door and its frame. The other side of the door was dark, but she thought she sensed fresh, moving air tickling her cheek.

With the help of a small craft knife, the door's latch opened with a satisfying click. Anna eased the door open. Before her stood a panelled screen, the kind to get dressed behind. She shifted it aside to reveal a bright and tidy room, with a mannequin stood guard in the corner, and a vase of flowers on the vanity table beside her. The room felt quite cold despite the sunshine spilling through the French windows to her left. She watched herself pirouette across the room in the mirrored doors of a huge wardrobe, and stifled a giggle - it wasn't every day she discovered a secret dressing room.

Though she couldn't think of how or why, the place had a cloying familiarity, reminding her of snowballs and fun times, prancing and twirling around like a snowflake, as she was doing now. But the memories were remote, like they belonged to someone else, or like she had dreamt it all a long time ago. And she was being ridiculous anyway, because snow doesn't fall indoors; her mind was obviously muddled. Just as Anna was gathering the courage to inspect the other doors, the flowers on the table caught her eye again. She reached out her fingers to brush against the delicate petals. They were frozen.

"Anna?"

Anna whirled round to face her discoverer, and her guilt evaporated into laughter to see that familiar, estranged face.

"Elsa! Sorry to intrude, I was just exploring, and, wow, this room is just lovely; has it always been here? I haven't seen you recently, have you been well? Last time I knocked was, hmm, a while ago, but that's not to say I haven't thought of you-"

"How did you get here?" Elsa interrupted, her slow deliberate words breaking off Anna's rapid torrent. She was not returning Anna's excited smile, but was instead holding her own arms with her gloved hands, like she was hugging herself.

"Um..." Anna started to realise what she had done, that she was trespassing, that she was unwelcome. "I found a door in my room, and I just..." She faltered. Elsa didn't even look angry. Her pale face, so like Anna's in shape and features but with a glacial beauty that Anna had never seen in herself, struggled to remain composed, yet the set of her mouth betrayed her fear - fear of Anna.

"You can't be here."

"I'm sorry-"

Suddenly another voice called from what Anna presumed was Elsa's bedroom. "Elsa, dear, what were you just saying?" It was their mother. Elsa didn't even turn her head. The sisters' eyes locked, and then Anna knew. Years of quiet, secret, repressed resentment that she had so successfully hidden from herself, all wrong. It was not her parents who wanted her separated from Elsa, but Elsa herself. The fear Anna had thought she had seen in Elsa's face clouded over with icy resolution. Like she had remembered herself, Elsa's back straightened, her chin tipped up and her hands fell to rest, clasped in front of her stomach, in a posture of regal self-possession.

"Leave!"

Anna couldn't disobey. She tore her eyes away from her sister, slipped back behind the screen and through the half-open door, swinging it shut behind her. Inexplicably shivering, she slid down with her back against its cool surface to sit on the floor, hugging her legs. Remembering how those peculiar frozen flowers had shrunk away from the heat of her hand, she tried to recall when she had wronged Elsa, burnt her with words or threatened her in any way. It was preposterous. Anna knew herself, knew she was uncomplicated and compassionate, knew she had never done anything in her life to become an object of fear.

She turned her head towards her bed. "I'd never hurt you," she whispered to the blonde doll in a blue dress sitting by her pillow. The doll just smiled at her and Anna imagined standing up, charging back into the dressing room and demanding an explanation, forgiving and embracing and forgetting a childhood of isolation at the sound of her sister's redeeming laughter.

She waited four years to hear it again.