About 7 minutes to read aloud.

The wind that night was strong enough to take the roof off a bicycle shed, and it took the letter too, tearing it out of a boy's postbox three streets from the sea and carrying it, rain-soaked and spinning, over hedges and chimneys and half of Norfolk before it came down, at last, in the chute outside the sorting room at the North Pole.

Mrs Wren heard it land. She always did. The chute had a particular clatter, like a coin dropped into a tin, and she had been sorting letters long enough to know the difference between one that had travelled well and one that had not.

This one had not travelled well. The envelope was grey with damp, and the ink on the front, wherever it had been written, had run into a soft blue smear. She could make out a capital S, and further down, in what might once have been a careful hand, the shapes of two more letters that could have been almost anything.

"No good," she said, to nobody, turning it over under the lamp. There was no house number. No street. No town. Just a smear, a corner of stamp, and rain.

She could have set it aside. There was a box for letters like that, high on the shelf, and every year a few went in and stayed there. But she opened this one instead, because the paper inside was dry, and because whoever had written it had gone to some trouble.

The letter itself was short. It said his name was Tom, that his dog was called Biscuit, that his gran's cottage had a crack in the bedroom ceiling shaped like a boat, and that from his bed he could hear the night train going past, always at the same time, always the same low sound, like someone breathing.

Mrs Wren read it twice. Then she stood up, took down the atlas that lived on the shelf behind her desk, and the ledger beside it that listed every railway line still running trains after midnight, and she began, patiently, to work.

A line that ran late. A cottage close enough to hear it but not so close as to shake the china on the dresser. A gran, which meant a smaller house near a bigger one, and a boy who came and went with the school holidays. She had three counties crossed off before supper, and the whole search narrowed to one stretch of coastline by the time the workshop bell rang for the night shift.

It took her four days in all, worked in an hour here and an hour there between the ordinary sacks of letters that did have proper addresses on them. She wrote to nobody and asked nothing, because there was nobody to ask. She built the address from the boat-shaped crack and the sound of the train and the name Biscuit, which turned out, in the end, to be the useful one. A dog barking at a level crossing at the same hour every night is the sort of thing a signalman remembers, if you can find the right signalman to ask, and Mrs Wren could not, not really, but a letter that has come from somewhere always leaves a trace of that somewhere behind it, if a person looks with enough patience.

On the fifth morning she wrote, in her own hand, on a fresh card, the address she had built: a cottage two doors along from a level crossing, on the lane behind the church, in a town whose name she was fairly sure of and a station she was certain of. She pinned the card to the letter and carried both through to the loading room herself, rather than trusting them to the usual chute, because an address built this way did not seem to her the kind of thing that ought to be dropped twice.

Nobody in the loading room asked how she knew. They had watched her do it before, with other rain-smeared envelopes and other half-legible names, and they had stopped asking, because the answer was always the same. She listened to what the letter told her about the house, and then she went looking for the house.

On Christmas Eve the sleigh came down over a stretch of coast where a late train was, at that very moment, rattling past a level crossing, and somewhere below a dog began to bark, the way a dog does when it hears something coming that it cannot quite place. The second cottage past the crossing had a crack in its bedroom ceiling shaped like a boat, exactly where the card had said it would be, and the parcel packed for Tom went in through the chimney and was found, the next morning, exactly where a boy who had written a very difficult letter to read might have hoped it would be.

The envelope itself, grey and smeared and long past being any use, went into a drawer at the North Pole with the others like it: a small collection of addresses nobody could read, and every one of them, eventually, found.