It was first published on 23 July 1910, under the title "Valentin Follows a Curious Trail", in the Saturday Evening Post, Philadelphia.

[1][2] Aristide Valentin, head of the Paris Police, is on the trail of the world's most famous criminal, Hercule Flambeau.

Valentin suspects that the arch-criminal is going to London to attend an international conference of clergymen, to possibly steal one of the precious religious articles on display there.

Valentin learns that the smaller of the two priests threw his half-empty bowl of soup at the wall before quickly leaving the restaurant.

When Father Brown refuses, Flambeau triumphantly reveals that he has already obtained the cross and slipped the priest a dummy package.

Father Brown replies that he switched the packages back at the sweetshop and mailed the cross safely to a friend at Westminster.

He explains how he suspected his companion was no priest because he recognized the bulge up his sleeve as the "spiked bracelet", a criminal insignia.

Brown rebuts the threat by illustrating that he has been committing acts to draw the attention of the police (throwing soup, knocking over apples, smashing a window) and leaving an obvious trail for them to follow.

The episode "La Croce Azzurra" of the RAI television series I Racconti Di Padre Brown (2011) is also an adaptation of this story.