The Hundred Years' War had paused. After the eleven-month siege that delivered Calais to Edward III in 1347, both kingdoms had signed the Truce of Calais and were tired enough that it kept getting renewed. But Charny, freshly placed in command of all French forces in north-east France in July 1348, kept looking at Calais. He had been one of the knights who, in 1347, had ridden up to Edward's siege lines and challenged him to come out and fight in the open field; the English had refused, the French had marched away, and the next day the burghers came out with their nooses. Charny did not forget. By mid-1349 he had a plan. The cheapest way to take a fortified city is to bribe a man who controls a gate. Calais's galley master, Amerigo of Pavia, controlled a tower over the harbour with a gate into the citadel. He was an outsider - Italian, low-born by knightly standards, easy to suspect of avarice. Charny reasoned correctly that Amerigo could be bought. He reasoned incorrectly that the buying would stay secret.

Edward III heard about the plot in late December 1349, possibly from Amerigo himself. He brought the Italian to Havering, near London, on Christmas Eve and made him a counter-offer. The English punishment for treason was elaborate - hanging short of death, emasculation, disembowelment, beheading, quartering. Edward offered to skip the entire program if Amerigo went along with a counter-plot. Amerigo's brother was kept in England as insurance. The king then sailed for Calais with nine hundred men - three hundred men-at-arms and six hundred archers - under the titular command of Sir Walter Manny, the first captain of Calais. The whole expedition was disguised. Edward fought in plain armour and under Manny's banner. He wanted the trap to look like a routine garrison response.

Charny did not bring a small force. He had assembled fifty-five hundred men at Saint-Omer, twenty-five miles from Calais - fifteen hundred men-at-arms (most of the senior knights of north-east France) and four thousand infantry. He picked New Year's Eve for the darkness, the low tide near dawn, and the chance that English sentries would be celebrating or asleep. A hundred and twelve men-at-arms would slip through Amerigo's gate at night, seize the citadel and the Boulogne Gate from inside, throw the gates open, and let the main force in. Leading the advance party was Oudart de Renti, a French knight who had been banished, joined the English, then re-defected to France - chosen by Charny for his knowledge of the area and to give him a chance to redeem his honour.

The advance party reached Amerigo's gate before dawn on 1 January 1350. The gate was open. Amerigo emerged, accepted his first instalment of bribe money, and led a small group of French knights into the gatehouse. A French standard was raised on the tower. More men crossed the drawbridge. Then the drawbridge went up, a portcullis dropped in front of the French still on the beach, and sixty English men-at-arms surrounded the party inside the gatehouse. All of them were captured. A trumpet sounded. The Boulogne Gate opened. Edward, in plain armour, led out his household troops and a detachment of archers. "Betrayed!" went up the cry from the French side, and a large part of Charny's force broke and ran. Charny held what remained and fought a hard rearguard action; Edward was given a serious fight. Then the Black Prince, Edward's son, led his own household knights out of the north Water Gate and along the beach, hitting Charny's left flank. The French line collapsed. Two hundred French men-at-arms were killed. Thirty knights were captured. Charny, with a serious head wound, was among them. So was Oudart de Renti.

Edward, conscious of his image, invited the higher-ranking French captives to dine with him that evening and revealed that he had fought them incognito. He was courteous to all of them except Charny, whom he taunted publicly for having abandoned chivalric principles by fighting during a truce and by trying to purchase his way into Calais rather than fighting for it. Charny, the future author of the Book of Chivalry, the keeper of the Oriflamme - the sacred royal battle banner of France - took the accusations in silence. They became standard English propaganda for years. He spent eighteen months in English captivity. He wrote much of his book there, warning the knights of France against the very cunning schemes he had just been caught in. Six years later, on 19 September 1356 at the Battle of Poitiers, Charny died holding the Oriflamme as the French army collapsed around the captured King John II - fulfilling, in the end, the oath of his office. The English kept Calais for another two centuries.