Imagine a world where nothing is required to live. You don't have to work, pay bills, or even worry about dying. You don't even know what death is. In this carefree world, you have a nice home, a soft bed, friendly neighbors, and green parks to stroll in.
In the Whistle Tube, such a world exists. It's called Heaven, and its creator is RIM, a bodiless being who plugs a group of human-like beings into his world with the absolute certainty they they're going to love it. He is stunned and perplexed, though, when their reaction is exactly the opposite. They hate it. They're miserable, so miserable they ask RIM if there's any way he can terminate their existence, which gives RIM an idea.
RIM creates a secondary world, a mortal world within which the beings will be conscious of their impending death, which they will perceive as absolute and final––an eternal descent into darkness. But in reality they'll be returning to Heaven upon their death.
As with all RIM's plans, nothing goes as he thinks it will. And he'll be forced to change course when the beings return from the mortal world with only one wish: to return to it over and over again.