The submission of the rebels

Aged 18 or 19, William had narrowly averted death. He first sought the support of his lord, the King of France, Henry I, who had not lost the opportunity to take advantage of the minor's age to resume control over their common frontier in Vexin.

William's loyal supporters were in the regions where ducal power had been well-established: Rouen, Evreux, Lisieux, Falaise and Exmes. The leader of the rebels was Guy of Brionne, son of the Count of Burgundy and Alice, daughter of Richard II. Guy had been brought up at the court of Normandy and was very well acquainted with William. He exploited the traditional docility of the lords of western Normandy, however, to his own advantage. The plot included in particular Néel de Saint-Sauveur, Viscount of Cotentin; Renouf de Briquessart, Viscount of Bayeux; Hamon le Dentu, Chatelain of Creully; Raoul Taisson, Lord of Cinglais, and Grimoult of Plessis. They all swore an oath on Holy relics at Bayeux to bring about the death of the Duke.

Following a period of troubles, above all marked by sieges and forays which were a feature of warfare at the time, the Duke led his army to face his enemies in battle. He was supported by the troops of the King of France.

The encounter took place at the Val-ès-Dunes near Caen in 1047.

Aided by the defection of one the plotters, Raoul Taisson, William and Henry I put the enemy army to flight. The rebels closest to the ducal court were quickly pardoned in exchange for their submission. Néel de Cotentin and Renouf de Briquessart had their status of viscount restored. Guy of Brionne chose exile. Grimoult of Plessis who was outside the ranks of the higher aristocracy was one of the few to be imprisoned and was probably executed.