Clio Marsh is enjoying a friendly dinner with a few neighbours when one of them, David, is shot in the back by an unknown intruder. The group had been celebrating David's upcoming competition in the dangerous sport of horse-eventing, in which he had planned to compete on the fierce and supposedly unbeatable horse Gilgamesh. While David lies incapacitated and bleeding, a valuable painting is stolen. Coincidentally, the painting is also called Gilgamesh.
As Clio's husband, Detective Harry Marsh, tries to piece together the facts, Clio forms her own theory about the motive behind the attack and robbery. Suddenly David's substitute is shot. Then Clio finds the missing painting in the most unlikely of places. So while Harry struggles to find the identity of the gunman, Clio begins her own ad hoc investigation. It seems that the key to discovering the perpetrator lies in understanding the chances of the proud horse Gilgamesh. Or so Clio thinks . . .