Head Cases (PAR Unit)

Mick Herron's Slow Horses meets David Baldacci's The Camel Club in a hot new series. When a serial killer starts targeting other serial killers, and seems to be one step ahead of the FBI at every turn, only one team of misfits has a chance of stopping him ...

Shortlisted for International Thriller Writers Awards 2026 - Best Series Novel

Gardner Camden is a walking analytical brain with an affinity for riddles, puzzles, and codes. It makes him the perfect fit for the Patterns and Recognition (PAR) unit of the FBI, a team of five brilliant but misfit agents who are too talented, too extraordinary to fire from the Bureau.

Gardner's smart, but he's all business - except for his seven-year-old daughter and occasional visits to his elderly mother, he prioritises his work and justice over everything else, no matter the cost.

A serial killer from one of Gardner's solved cases, presumed to be long dead, is found murdered, and then soon after, another body with a similar story. The mastermind murderer has left clues and riddles for Gardner and his team - a mathematician, a sniper and weapons expert, a computer analytics specialist, and their leader, a career agent - as they track him across the country. With the threat of PAR dissolving, the team can't afford to make any mistakes and Gardner must work to solve the riddles before it's too late.

McMahon's suspenseful and irresistible thriller confronts the scales of justices in the face of a twisted vengeance plot as this engaging cast of characters works together to stop a misguided vigilante.

'McMahon's inventive, eventful, smartly written, Head Cases, with its brilliant but fallible protagonist, its twisted-minded perpetrator, and its mix of sentiment and intrigue, is a terrific debut that augurs well for further installments in the series' Wall Street Journal

'[A] sterling series debut … McMahon introduces several clever wrinkles to this classic cat-and-mouse setup, while making the socially awkward Camden and his colleagues three-dimensional enough to sustain future installments. With pulse-pounding action and enough surprises to blindside even seasoned mystery fans, this is an excellent start' Publishers Weekly starred review

'Everything this crime fiction-lover could want: gripping, twisty, and atmospheric, with a harrowing and intriguing plot, gorgeous prose, and fresh and compelling characters' Karen Dionne, author of the #1 international bestseller The Marsh King's Daughter

'A masterfully done game of cat and mouse with an unforgettable cast of characters. I couldn't get enough of this addictive, propulsive story of intellect and intrigue' Matthew Quirk, author of The Night Agent