On an Orkney isle, siblings Isla and Sandy survive by hunting seabirds, retelling the stories of their ancestors, and defending the isle from dreaded "Incomers". Their world is upended with the arrival of an awkward council worker who has come to uproot them from their homeland.

The Harroway's mad brother, Martin, has died. Amongst his possessions is an illustrated children's book containing a strange warning about one of the characters within it: Once you've seen John, he can see you. And he's coming to get you.