This time around, Caffery's Major Crime Investigation Unit and Marley's underwater unit (a unit used for things besides fighting underwater crime) team up to track down a serial kidnapper of children before he kills -- or kidnaps again.
Caffery is his usual tortured, dogged, intuitive, hyper-competent self, still estranged from dogged, intuitive Marley due to a misreading of an event in a previous novel. The strengths of the two characters will ultimately need to combine in order to save the innocent and stop the guilty.
Hayder goes all-in on the details and minutiae of the police procedural here. She occasionally over-does the exactness (you'll never read a novel in which the term 'fuller's earth' appears more in a 50-page span unless there's a novel in which 'fuller's earth' is the protagonist). But for the most part, this is detailed thriller that establishes a sense of verisimilitude when it comes to both police work and the anguished reactions of the parents whose children have been stolen.
The plot twists and turns and twists again. The identity of the kidnapper may occur to you before it's finally revealed, so don't spoil it for anyone else. Only Hayder's occasional desire to slip into the quasi-mystic mars the novel -- a final necessary revelation seems to arrive by psychic fiat, and the recurring character of The Walking Man is a straining for mysterious effect that Gone neither needs nor benefits from. But overall, highly recommended.