One need not read The Bone Clocks to understand, follow and enjoy Slade House, though the world in which it's set will be familiar to those who who have read the other novel.
Through five chapters set in five decades starting in the 1970s, we are sucked into a house that seems haunted and pretty scary. At first I was sure only "bad people" were invited, but that doesn't seem so clear by the end. Each story follows a similar path, yet they all advance the novel further and the stories are not just linked but the final few are completely interdependent.
The basic gist is that a pair of twins seem to live in a house that can only be accessed every nine years, when a door appears to those invited, and all those invited will become the twins' victims.
Why? Well in the most obvious sense, the twins need to feed on these people's souls. And since David Mitchell is the author, it's every nine years. I can accept that. But there are bigger questions for me -- why these people? Luckily one of the characters asks, and the answer is less than satisfactory:
"What does 'deserve' have to do with anything?" Norah Grayer lifts her sharp eyebrows. "Did the pig whose smoked flesh you ate at breakfast 'deserve' her fate? The question's irrelevant. You desired bacon and she couldn’t escape the abattoir. We desire your soul to power our operandi, and you can’t escape our lacuna. That’s it."
So I don't get any metaphysical, moral or theological answers from Slade House, but it is great fun.