New York, winter of 1873. Irene is thinking about an event that happened in London, months ago, with her good friends Sherlock and Lupin. Memories unfold quickly between the meeting with a man who tells a strange story, a crime without a body, and the discovery of a network of tunnels beneath the city. A true underground labyrinth, where the three children conduct their investigations, step by step, following their intuition, into the unknown and dangers, in yet another unforgettable, as it will prove, adventure.