The Dagobert Conspiracy. Chateau Sarony, #11

Aggie Cleburne and Colin Burstow, both serving US Marine Corps officers, decide to take their summer 2016 leave in Europe with their friends Anna and Martin Price at Chateau Sarony close to Dijon. The visit will also enable them to attend the wedding between Nikki Prendergast, who works for Anna and Martin's company Timewarp Ltd and Gendarme Capitaine Jean-Paul Ricard. When a local dam is breached, and sabotage by members of the US forces based at Dijon suspected, duty interrupts their vacation plans.
Along with Anna and Martin, they become involved in what at first appears to be the discovery and theft of a long-lost item from the French Crown Jewels, last seen in 1795. Two subsequent murders then lead them to conclude that the matter is connected to a conspiracy which could result in the loss of hundreds of innocent lives and cause irreparable damage to relations between France and the United States.