Aristide Valentin, head of the Paris Police, is on the trail of the ingenious criminal and master of disguise Flambeau. Valentin suspects that the Flambeau is going to London to attend a conference of clergymen to steal the precious religious articles on display there. While traveling to London, Valentin encounters a Catholic priest, and he overhears Father Brown tell a lady that he is carrying the "Blue Cross", a sterling silver cross covered in precious blue stones. Valentin warns Father Brown that it is dangerous to tell anyone that he is carrying an object of immense value. Valentin decides to tail Father Brown but loses him and there begins a desperate chase to avert a terrible crime.