Every night at exactly midnight, someone knocks on a door that should not exist.

Twelve-year-old Milo Farrow is not looking for an adventure. Since his father died, all Milo wants is for the world to be ordinary and reliable and exactly as advertised. Then he cuts through the old goods yard one October evening and sees a green signal lamp burning above a railway station that has been closed and demolished since 1963.

Inside, there is a door with no handle, no keyhole and no hinges. It is warm to the touch. And at midnight, something on the other side knocks three times.

When Milo finally answers, he finds a man who has his face, his handwriting and twenty extra years, and who has time to say only eight words before the door closes:

Whatever happens, don't let the Keepers find you.

The Keepers of the Impossible have been protecting the world from things like that door for eleven hundred years. They are polite. They are patient. They keep careful records. And they are very good at making four thousand people forget an entire week.

But the warning was never really about them. Something far older has been waiting at doors like this one since 1891, it has never told a single lie, and it is about to offer Milo the one thing he wants most in the world.

All he has to do is decide who pays for it.

A gripping middle-grade mystery about time, memory, friendship and the price of getting what you wish for.