Herlews first appears while Arthur is ill in a pavilion set in a meadow. The king hears a rider approaching in great sorrow and asks the cause, but the knight refuses to explain, saying that Arthur can do him no good. He continues towards the castle of Meliot. Arthur sends Balin, the knight associated with two swords, to bring the stranger back by persuasion or force. The episode begins with an unanswered question, and nothing in Malory's immediate account establishes the knight's family, home or original reason for grieving.

Balin catches up with Herlews and an unnamed damsel in a forest. Herlews resists the summons because returning will harm him without helping the king. Balin insists, but also says that he would rather avoid a fight. Herlews then asks whether Balin will answer for his safety, and Balin promises to do so even at the cost of his own life. On that assurance, Herlews leaves the damsel and accompanies him back. The agreement gives the later killing its force: the victim has accepted protection, not sought a combat in which he knowingly risks defeat.

As they come before Arthur's pavilion, an invisible attacker strikes Herlews through the body with a spear. The wounded knight identifies the killer as Garlon and tells Balin to take his horse, which is better than Balin's own. He must return to the damsel, follow the quest she was guiding and avenge the death when he can. Balin swears to undertake it. Arthur gives Herlews a rich burial and has the treachery recorded on the tomb. The damsel preserves the broken shaft of the spear, turning the murder's physical remnant into a continuing reminder of the obligation.

Balin and the damsel travel onward, and another knight, called Perin de Mountbeliard in the consulted Malory text, joins them. Garlon kills him while invisible as well. At a later lodging Balin encounters a household whose son has also been wounded by Garlon, and learns of a feast at King Pellam's castle where the murderer may be found. Herlews is therefore one victim among several whose injuries draw the pursuers together. His death is not the first violent act in Balin's whole story, but it is central to this particular chain of vengeance.

At Pellam's feast, Balin recognises Garlon and is struck by him after staring. Balin responds with his sword, then asks the damsel for the spear fragment and drives it through the man who used it against Herlews. Pellam attacks to avenge his brother, and Balin's sword breaks as he defends himself. Searching for another weapon, Balin seizes the sacred spear associated with Christ's Passion and wounds Pellam, bringing down the castle in the Dolorous Stroke. Herlews has no further scene: his name survives through the tomb, the entrusted quest and the fragment carried until vengeance is achieved.