The story attempts to offer a scientific explanation for lycanthropy. One of the characters, a psychiatrist named Carl Lundgren, states that the condition is the result of a mutation in the pineal gland of the afflicted person's brain. At the same time Blish also relies on established werewolf mythology for plot devices, such as the use of wolfsbane and silver. A female character named Doris is revealed to be a witch, and this subplot is important to the story's resolution.
The story takes place on a manor in the Scottish countryside. A wealthy man named Tom Newcliffe has invited a group of artists and friends together for a weekend gathering. Of particular importance to the story is Jan Jarmoskowski, a Polish concert pianist. Paul Foote, a painter, detects hints that Jan may in fact be a werewolf, and he is proven right. The musician transforms under the light of the full moon, and the guests at first prepare some makeshift silver bullets and attempt to track him, but this results in nothing more than a dead hunting dog and some wasted ammunition. Following this, the group fortifies itself inside the mansion to wait for their quarry to return, at which point they hope to be able to defeat him.