In the near future, the ice caps have melted and the sea level has risen making much of the world uninhabitable. The inhabitants of an island nation have built a large, concrete wall around the perimeter of the island to keep out unwanted immigrants, aka environmental refugees. Every citizen takes a turn in their life standing atop the wall as a defender, guarding the island all day, or all night, with very little in the way of comforts. If a defender fails to keep a refugee out, the defender will be set out to sea. I thought sitting on the cold wall, watching the sea for “invaders” was going to be the whole story, sort of a post-apocalyptic waiting for Godot, but the book eventually turns into a suspenseful survivalist quasi-thriller. Still, I thought the bread and butter of the story was the time the main character lived on the wall as a defender. Lanchester writes the desolation and loneliness of sitting on a cold hunk of concrete very well.