Lanceor appears when a damsel brings a sword to King Arthur's court. The weapon can be drawn from its scabbard only by a knight who meets its exceptional conditions. Balin, recently released from imprisonment and poorly equipped, succeeds where the more splendid knights have failed. Malory expressly connects Lanceor's resentment with this unexpected honour. The episode concerns a sword worn by the visiting damsel, not Arthur's earlier sword in the stone. Rank and appearance have failed to predict who will achieve the adventure.

The court is then shocked by Balin's killing of the Lady of the Lake. She demands his head, while he accuses her of responsibility for his mother's death. Arthur condemns the killing of a woman under his protection and orders Balin away. Lanceor asks leave to pursue the offender, and the angry king allows him to make the attempt. This provides a public grievance alongside Lanceor's private envy. He arms himself, follows rapidly and catches sight of Balin on a mountain road.

Lanceor demands combat in the name of Arthur's injured honour. Balin regrets the prospect of fighting another member of the king's court but cannot persuade him to abandon the challenge. They charge with their spears. Lanceor's weapon breaks against Balin's shield, while Balin's spear passes through his opponent's shield, armour and body, reaching the horse's croup. Balin turns and draws his sword before realising that the other knight is already dead. The joust has ended at once, without the prolonged exchange Lanceor's confidence might have suggested.

Colombe reaches the scene after the combat. Her lament binds her loss to Lanceor's death, and she takes his sword. Balin tries to remove it without injuring her, but she suddenly kills herself. The narrative records his horror, remorse and inability to prevent the act. Her death is part of the tragedy, not a model of devotion that readers must imitate. Soon afterwards Balin meets his brother Balan, while a dwarf arriving from Camelot warns that the dead knight's relatives will seek vengeance.

King Mark of Cornwall happens upon the bodies and refuses to depart before arranging their burial. A rich tomb is found in a church and brought into use at the place of their deaths. Its inscription names Lanceor as the Irish king's son and Colombe as his lady and lover. Merlin adds the names of Lancelot and Tristan in gold, predicting their future combat there. He also rebukes Balin over Colombe's death and foretells the Dolorous Stroke and misery in three kingdoms. The prophecy does not make the tomb itself a verified monument at Camelot, nor does it name Pellam in that speech. Lanceor's short career survives through the consequences that gather around the shared grave.