New York, winter of 1873. Irene is reflecting on an event that took place in London, months ago, with her good friends Sherlock and Lupin. The memories quickly unfold between meeting 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 instincts, into the unknown and dangers, in yet another unforgettable adventure, as it will turn out.