John and the gang go back to school to investigate the suspicious death of a headmaster. In doing so, John is faced with his own bad memories of school but is pleasantly surprised when he reconnects with his old mentor, the man who taught him everything about puzzles. With John distracted by the case, Lucy follows a new lead which she hopes will bring her closer to finding James. Lucy's covert mission takes her to Wales, but she's not the lone wolf she thinks she is, and her discoveries leave her rattled. A shocking revelation from IT consultant Holly forces John to question everything.