"The Blue Cross" is a short story by G. K. Chesterton. It was the first Father Brown short story and also introduces the characters Hercule Flambeau and Aristide Valentin. It is unique among the Father Brown mysteries in that it does not follow the actions of the Father himself, but rather those of Valentin. 

Aristide Valentin, head of the Paris police, is on the trail of Flambeau, a notorious French criminal and master of disguise, believed to be in London. Valentin follows both his instincts and a series of clues, but is baffled by the intervention of a strange little priest...