From the back of the book:

Spiral dust is a transcendent, wonderful, and orgasmic high. That's what users claim, eyes wide to reveal blue fractals instead of irises. Spiral dust addiction is on the rise. But why? Who is the Dustman who deals this strange substance from the shadows? And why do heavy spiral dust users tend to disappear without a trace?

In The Dark Spiral, the player characters are operatives of the Estate, a secretive organization pledged to protected the Earth. A mission to shut down a local dealer of spiral dust takes them farther than they expected when the trail leads to alternate, limited worlds called recursions where the rules of reality are different.

Game masters can intermix secondary Estate missions (in any order) with the main story arc of the adventure to create a mini-campaign. The Dark Spiral introduces players to the wonders of translating between alternate worlds of The Strange and showcases the narrative structure of the game.

In addition to introducing new locations new covered in The Strange corebook, The Dark Spiral offers new creatures, characters and items to add to any campaign.

- description provided by the publisher