Following the break-up of the Carolingian empire, Nominoe, Duke of Brittany, rebelled against the authority of Charles the Bald and attempted to expand into Frankish territory.
Erispoe's troops had arrived at Messac and were in the process of crossing the Vilaine when Renaud launched his surprise attack.
Sources differ about what happened next, though all agree that the tables were turned on Renaud by the timely arrival of Erispoe's ally Lambert II.
According to one version of events, Erispoe was saved at Messac itself by Lambert, who had gathered soldiers on the borders of the Anjou, and had advanced on the Vilaine to join his forces with Nominoe.
The Frankish soldiers were lying on the grass near the edges of the river Isac, in complete abandonment, when suddenly Lambert's and Erispoe's armies attacked and destroyed them.