A magical serial killer is stalking the Occult town of Wrackton. Hypnotic whistling causes victims to chew their own tongues off, leading to the killer being dubbed the Whistler (original, right?).

Enter the Undetectables, a detective agency run by three witches and a ghost in a cat costume (don't ask). They are hired to investigate the murders, but with their only case so far left unsolved, will they be up to the task?

Mallory, the forensic science expert, is struggling with pain and fatigue from her recently diagnosed fibromyalgia. Cornelia is suddenly stirring all sorts of feelings in Mallory. Diana is hitting up all her ex-girlfriends for information. And not forgetting ghostly Theodore: deceased, dramatic, and also the agency's first - unsolved - murder case.

With bodies stacking up and the case leading them to mysteries at the very heart of magical society, can the Undetectables find the Whistler before they become the killer's next victims?