Arthur receives a summons from the Armorican kingdom of Benoic, to which he swore an oath to be the kingdom's champion, for assistance against the invading Franks. Unable to go himself, he sends Derfel with 60 men. Derfel is to join forces with Arthur's cousin Culhwch and to write to Arthur if more men are necessary. Before he leaves, Sagramor warns Derfel of the Edling of Benoic, Lancelot, saying he can be treacherous. Upon arriving, Derfel is taken to Ynys Trebes, the island capital of Benoic. There he meets King Ban, who is upset when he learns that Arthur is not coming but is delighted upon learning that Derfel is a literate warrior. Ban shows Derfel the library of Ynys Trebes, which is overseen by the foul-tempered Father Celwin. At dinner, Derfel meets Lancelot and the two instantly dislike each other. Derfel is only prevented from beating Lancelot senseless by the intervention of Galahad, Lancelot's half-brother. Galahad explains in private that Lancelot will not take defeat lightly and suggests that Derfel leave Ynys Trebes immediately. Derfel returns to shore accompanied by Galahad, who wishes to fight alongside him.