Returning to London with her father, Irene doesn't have time to enjoy reuniting with her good friends Sherlock and Arsen, as a sudden event disrupts the fragile peace of her home: Horatio Nelson, the valuable butler of the Adler family, disappears, leaving behind only a brief note. Irene is sure that something suspicious lies behind this unexpected move and decides to investigate it with the help of her friends. The three children thus find themselves confronted with a story involving a mysterious crime in the Docks of London, an authentic enigma with roots in the distant and fascinating colonies of India.