There was a small whoosh of air, then the doors opened and the silence of the carriage was shattered by the roar of the station. A herd of haggard men, tired women and excitable children poured out onto the platform, chattering away, caught up with the rush of their lives and not once pausing to notice a small, skinny girl peering out from behind one of the carpet-covered seats. The little girl waited silently until they had all gone, then swung her rucksack over her shoulder and nervously hopped off the train. She wove her way down the platform to the gates and stood huddled behind a column until she could slip through the barriers without the guard noticing. She hurried quickly over to the wall of a shop and hunkered down on a metal ledge that jutted out near the bottom. If anyone had been watching her, they would have thought that this was a girl who certainly did not want to be seen.

The curious thing was, Clara Darnell had no real need of hiding behind columns or sneaking around. Ever since she was little, she had had the unnerving ability to go unnoticed in almost any situation, even when someone was looking right at her – it was almost as if people just forgot that she was there. This in itself was also a curious thing, as Clara's bright white hair, chalk-white skin and otherworldly blue eyes didn't exactly lend themselves to disguise.

So Clara sat, unseen, on that ledge, watching the shoes of strangers walking by. Grubby trainers, smart patent heels, businessmen's brogues, all paraded by in a never-ending catwalk of monotony. Clara hunched up her knees and rested her chin on her elbow. People could be so boring sometimes, wearing a uniform even if they didn't realise it. Unusual herself, Clara had always been attracted to unusual things, and spent most of her free time trying to find them. So far, her best catch was a surgeon dressed as a burger who tried to beat up a lamp-post with a carpet.

Unfortunately, King's Cross didn't seem to be very interesting this morning. Clara sighed, and was about to go to find a new watching post when a pair of bright purple, perfectly bizarre, crocodile-skin loafers with giant silver buckles strode past. Clara looked up. This was it! Heaving up her tattered rucksack, she swerved to avoid a gaggle of giggling schoolgirls and hurried off after that so unusual pair of shoes.

The shoes' owner turned out to be an elderly lady wearing a green dress and a hat with a stuffed vulture on it (even better!). She was busy berating a tubby boy of about eleven, occasionally giving him a whack with her big, red handbag for good measure. The boy was pushing a trolley with a huge box on it and was clutching – was that a toad? Wow, this day was really perking up! Clara sidled along behind them, blending in and trying to look innocent.

"What did I tell you about double-checking your list? Sometimes I wonder if your great-uncle Algie wasn't just seeing things when you bounced that day – surely no-one as idiotic as you could ever be a wizard! And what do you have to say for yourself, hmm?" The boy winced as she hit him again with her handbag.

"I'm sorry, Gran. It's only a blanket though, I'm sure I'll manage withou- ooofff!" A double hit, right in the stomach.

"It's not the point if you can 'manage without it' – Enid knitted it especially for you and she'll be very upset. Now I'll have to deal with her making exploding soup for weeks! You'll have to send her an owl apologising, and it had better be a good one! Right, we're nearly there. Smarten yourself up, Neville, you're looking awfully scruffy!" Neville grimaced and tugged down on his jumper.

Well now, this was beginning to get a little bit more than interesting! Were these people crazy? Neville and his Gran shuffled through the ticket gates despite not seeming to actually have a ticket, and Clara followed closely, squeezing through behind a woman with a pram. The pair continued bickering as they walked down the platform.

"I'm sure it was this one, Neville," said the old lady, poking a brick column between platforms 9 and 10. Clara gaped at them, her skulking forgotten – the old woman's whole hand had disappeared through the wall!

"Yes, this is it. Right then, you first, young man. Take your trolley." She prodded the boy and he nervously pushed the trolley up to the wall. "Quickly, boy, you're being conspicuous!"

With another whack of the handbag, the boy tripped through the wall and was gone. Glancing round at the oblivious commuters, the old lady followed him. Clara continued to gawp at the wall for a while before she gave her head a little shake and stepped through. She had a feeling that what she would find was going to beat the burger-surgeon hands down.