Lieutenant Colonel Hamish Hamilton is starting to enjoy his forced retirement from Military Intelligence. He lives an idyllic life of solitude in the seaside town of Largs on the west coast of Scotland, where the most exciting thing on his calendar is guessing the day's weather before going for his walks along the promenade. It's a peaceful existence away from firebreathing, man-eating, blood-sucking mythical creatures seemingly Hell-bent on ruining his quiet retirement.
But when Kelpies are suspected of kidnapping teenagers on the Isle of Cumbrae, and his former colleagues have disappeared, believed Killed in Action; a not uncommon situation in their line of work, he gets dragged reluctantly back to solve the youth's disappearances.
General Maxwell, head of the department, promotes Hamilton to colonel as a sweetener and assigns him an Afghan War veteran with metal teeth and PTSD as a bodyguard. Hamilton realises that the young captain who turns up on his doorstep the next day is to be trained as his replacement. If Hamilton wants to return to his retirement, he had better solve the mystery and educate the young officer in the ways of the Boundary Walkers.