Alan Henderson hated his grandfather. But his grandfather, Sir Reginald van der Helsing, wasn’t the only one he had come to hate. He fired a pebble at a passing bird but missed. An inaudible profanity quietly rumbled beneath his breath. A larger stone was chosen, and he reloaded his slingshot. His eyes followed his next prey. Aim readied, he waited patiently.

The turret in which he stood, an architectural feature of the family estate home, provided a lofty vantage point and unhindered solace...