In an alternate version of London in 1895, a few talented people can travel instantly between locations through Thresholds - tears in the fabric of reality. Lord Henry Lewis is a Warden, working to close dangerous Thresholds to prevent the sickness that flows from the darkness outside reality. William Marsh loses his family to the sickness when several undirected Thresholds are opened in his home. Marsh has the born talent to open Thresholds, and Lord Lewis's superiors order him to train the young man. Their relationship is strained, but they slowly come to rely on each other. The number of rogue Thresholds is increasing and times are dangerous. Lewis and Marsh discover that a cult, disillusioned by the scientific advances of the age, is opening the rogue Thresholds to destroy mankind. As London falls into darkness, they must race against time to stop the end of the world.