The search for the swords brings Douglas and Margaret back to England, where Mystere has hidden the sword Almace -- his own sword. Four months pass with no signs of Mystere, and Douglas and Margaret start to make plans to go in search of him. Then they accidentally become the caretakers of an injured street urchin. The boy - a sweep's apprentice - tells them that there is something invisible haunting the rooftops of London, preying on the children who make their living there.
In the absence of Mystere, Margaret insists that they need to help, but Douglas refuses. Margaret defies him and goes anyway, only to discover that what lurks on the rooftops is far worse than she had imagined -- Caedda has taken the rooftops of London as his domain. Worse, he has taken Mystere prisoner, he holds the sword Almace, and he knows that Margaret is there.
Trapped on the roofs, unable to reach Douglas, Margaret has to rely on her wits and her memories in order to save Mystere, herself and her unborn child.
