England, 1889.

Lady Jessica McAlpin is the commander of The Black Diamonds, the Great British Empire's covert monster-hunting division.

With her crew of criminals, misfits and desperadoes, Lady Jessica soars above the land in the airship HMS Zephyr, tracking down the diabolical foes of the Empire.

When events in London take a sinister turn, the future of The Black Diamonds looks to be in doubt. Captain William Flint, the half-vampire second-in-command of the HMS Zephyr, takes matters into his own hands. Plagued by devouring darkness and memories of a terrible past, he embarks on what will be his most dangerous mission yet: breaking into the Tower of London, the impregnable fortress at the heart of the Empire.