Throughout his young life, there were three things that Will knew to be completely, one-hundred-percent true: his mother had loved him more than anything in the world; Papa had magical powers and was thus the only person in the universe capable of keeping Daddy in the right amount of trouble on a regular basis; and Father Christmas was real.

Everyone had gotten him all wrong, of course, thinking he was some fat, white-bearded, biscuit-hungry stranger with a penchant for well-intentioned trespassing and travelling with companions of less than the statistical average height. The real Father Christmas wasn't like that at all. He was old, yes, older than Uncle Mycroft even, but clean-shaven and quite slim. He frequently travelled with a companion, but she was as tall as Papa, quiet, beautiful, and she would speak to Hamish in French. Father Christmas wasn't particularly jolly, instead viewing the world at all times with an odd sort of detached pleasure. From time to time, Will would catch his watchful eye and see a familiar smile on his lips. It was the same smile that Daddy reserved for Papa, a smile that seemed to say, I can't believe you happened to me.

No matter how old he got to be, Will would always go to him when he saw that smile, hugging him tightly and kissing him on the cheek.

Come to think of it, a number of things about the mysterious visitor were vaguely similar to their father: his eyes, his hands, his stride. This would all become clearer to Will much later in life, but it didn't seem significant when he was small.

Father Christmas sent them postcards throughout the year from places all over the world. Hamish insisted on tacking them to the walls of their room, and they now circled the ceiling in place of crown moulding.

'Daddy,' Will had asked once as their father added the newest addition to their growing collection, 'Why does he always sign as, "Uncle George"?'

'George Scott. It's a pseudonym.'

'What's a sewduhnimm?'

'Pseudonym. It's a name you use when you're pretending to be someone else.'

'Like when you say you're Uncle Greg?'

'Who?' Hamish giggled beside him. 'He's undercover, darling. If he signed them with his real name, he'd have to send cards to everyone in the world. This way, it stays our little secret.'

Hamish and Will agreed this explanation was most satisfactory.

Sometimes he appeared without warning. Daddy would bring them home from school and he'd be sitting on the sofa: big as life and rumpled and delighted to see them. He'd stay the night or, on occasion, the week, retelling impossible adventures and smoothing the lines on their father's face. And Papa would smile, his hand lingering on Daddy's back whenever they passed, soft words murmured in the kitchen. Sometimes he would disappear for ages: no visits, no postcards, no word at all. Will wondered if the quiet strain in the flat was a result of his prolonged absence. But without fail, he would always appear again: a gentle knock at the door and Hamish's shout of joy, late in the evening on 23rd December.

It was Father Christmas who taught Will and Hamish the Secret Code, the magic number that never failed to soften Daddy's stiff shoulders and bring a smile to his face. The numbers he always recited back, no matter where they were or who happened to be around: at home or at the Yard or piled in Uncle Mycroft's car.

'But what does it mean?' Will had asked him when he was very small. '"Seventy-six." It's got to mean something.'

He'd taken his time to answer, and Will adored the crooked, speculative smile on his face. 'It means everything important and everything good. It means, "I love you," and, "I miss you," and, "I'm so glad you're in my life". It means you're there for him and you know he's there for you and nothing will ever change that.'

'But it's just a number!'

'And this is just a flat in London. And you're just a little boy. But you're so much more than that, aren't you?' He grinned, and Will felt warm and safe and better than he thought he was. 'Anything can be special if someone thinks it's special.'

'I'm not special.'

'You're the most extraordinary thing in creation. At least you are to me.'

That wasn't possible, it couldn't be. But Father Christmas had a way of making impossible things come true.