From the back of the book:
THE CITY LIVES.
THE CITY SPEAKS IN DOORS AND PATHWAYS.
THE CITY IS DYING.
As dozens of doors to a strange land between dimensions begin to open throughout the Heart, the delvers step through to find the city of Elsewhere in turmoil. The lights which keep them safe are going out one by one. An ocean of vicious darkness roils at the edge of Elsewhere and seeps through to the City Beneath.
Will the delvers save this nexus of worlds?
This is a scenario supplement for Heart: The City Beneath. A copy of the core rulebook is required to play.