During a pastry competition that brings Italy's most famous pastry chefs to Gubbio, a young industrialist, promoter of the competition, is poisoned by a slice of cake "stuffed" with strychnine. The police investigations focus almost immediately on the ex-girlfriend, the beneficiary of a billion-dollar insurance policy. Thanks to Don Matteo's help, it will be discovered that the industrialist's death was actually caused by an "error" in person: one of the confectioners in the competition had to die. And it is in a secret recipe that the solution to the mystery is discovered...
