Free communication existed, in consequence of the truce, between Calais and the surrounding country, and Jeffrey de Charny, the governor of St.Omer, and one of the commissioners especially appointed to maintain the truce, opened communications with the Lombard captain.

Deeming that like most mercenaries he would be willing to change sides should his interest to do so be made clear, he offered him a large sum of money to deliver the castle to the French.
The Lombard at once agreed to the project.

Jeffrey de Charny arranged to be within a certain distance of the town on the night of the 1st of January, bringing with him sufficient forces to master all opposition if the way was once opened to the interior of the town.

It was further agreed that the money was to be paid over by a small party of French who were to be sent forward for the purpose of examining the castle, in order to ensure the main body against treachery.

As a hostage for the security of the detachment, the son of the governor was to remain in the hands of the French without, until the safe return of the scouting party.
Several weeks elapsed between the conclusion of the agreement and the date fixed for its execution, and in the meantime the Lombard, either from remorse or from a fear of the consequences which might arise from a detection of the plot before its execution, or from the subsequent vengeance of the English king, disclosed the whole transaction to Edward.
The king bade him continue to carry out his arrangements with De Charny, leaving it to him to counteract the plot.