Hugo Duchamp Investigates: The fourth trilogy' contains the tenth, eleventh, and twelfth books in the series following Hugo Duchamp, a Frenchman living and working as a policeman in London who is about to have his world turned upside down.

A horrific accident in 2010 triggers a chain of events in present-day Montgenoux (Prisonnier Dix) (Prisoner Ten). When the body of a young medical intern is discovered on the grounds of a scientific research institute, Hugo discovers she had a very difficult past, and the investigation leads to a family torn apart by a deadly secret and an evil murder. Hugo and his team must race against time to protect the innocent before a deadly killer strikes again.

After tragedy strikes at Baptiste Beaupain's school, Hugo agrees to go with him to the seaside town of Beaufort-Sur-Mer (Le Bateau au fond de l'océan) (The boat at the bottom of the ocean), where he is immediately thrown into a murder investigation. He is also reunited with faces from his past and together they must work together to fight a murderer who will do anything they can to keep their identity hidden.

In the mountains of France, a group of travellers reunites after forty years to once again walk the Camino de Santiago together (Camino de Compostelle) (The Way of Compostelle). However, when a horrific storm strands the group in a small village, one pilgrim seizes the opportunity to exact revenge in a deadly way and Hugo Duchamp must undertake a perilous journey to rescue his loved ones before the murderer strikes again.