Bobby Holmes, his cousin Brenda Watson and friends are embroiled in a deadly mystery in the North York Moors of England. An old beggar warns Bobby to stay away, and another stranger appears to be at the center of it all.

Bobby and his mates travel to the seaside town of Whitby, where a puzzling tattoo on the stranger is revealed to mean Wolf Slayer. Their goal, to track him down, leads them to baffling clues: the appearance of a group of gypsies and a librarian attached to Her Majesty's Government, who is researching a group of super wolves. His research dates as far back as King Edward and his ally, Peter Corbet, who is charged with ridding the country of these beasts.

Searching for his mates, friend Michael gets attacked and captured by the monster, then taken to the witches who control the creature.

Seeking their friend, Bobby and the others locate the gypsies, discover their leader is the beggar who initially warned Bobby, and receives aid and information. They learn that the mysterious stranger they've hunted is a descendent of Corbet, named Alex. Their new friend takes them to the Red Lion Inn for help in finding the cottage of the Witches of Westerdale.

They find it, burn the cottage along with the witches, rescue Michael, and return to the Inn. Here they find the beast, waiting. It is killed by Alex, who then leaves to help another in New Zealand.