Harpin's story begins with a demand that the unnamed lord will not accept: the giant wants his daughter. When her father refuses, Harpin attacks the household through its sons and possessions. He captures six young men, kills two before their father's eyes and threatens to kill the other four unless their sister is delivered to him. He also threatens to hand her over to his servants for abuse. The surrounding countryside has been stripped by his raids, leaving the fortified household with little protection beyond its walls.

Yvain arrives with his lion and notices the uneasy mixture of welcome and grief among his hosts. When he asks for the cause, the lord explains the threat and the approaching deadline. The family's expected defender is Gawain, one of King Arthur's knights, whose sister is the lord's wife, but Gawain is absent during the crisis surrounding Guinevere's abduction. The threatened daughter is therefore his niece and the captives his nephews. Yvain's friendship with Gawain strengthens his commitment to the household, but he cannot forget the separate promise already made to defend Lunete.

On the following morning Yvain waits anxiously, knowing that delay could cost Lunete her life. Harpin eventually appears with the four captive brothers, bound and mounted on miserable horses. A dwarf accompanies them and beats them with a knotted scourge. The giant carries his great pointed stake and repeats his demand for the daughter. The family begs Yvain to remain, while the knight weighs the danger of abandoning either obligation. He arms himself and rides out before the castle to face Harpin without surrendering the other task.

Yvain's lance wounds the giant, but Harpin's heavy blows with the stake force the knight down towards his horse's neck. Yvain draws his sword and cuts Harpin's cheek. The lion then attacks, tearing through the bearskin and wounding the giant's thigh. Harpin tries to strike the animal, but the lion leaps back and the blow misses; in the consulted French-romance translation, its paw is not broken. Yvain follows with decisive sword-strokes to the shoulder and body, and the giant falls dead. Harpin keeps using the stake rather than changing to a sword of his own.

The household's four surviving sons are released, and their sister is no longer subject to Harpin's demand. Yvain cannot remain to celebrate because the promise to Lunete still requires action. He asks the rescued family to take the dwarf to Gawain when he returns and report what the Knight with the Lion has done. Related English and Norse retellings carry the giant episode into other literary traditions, with names and details changing between versions. Its central function remains clear: Harpin creates a crisis in which rescue, friendship and the keeping of a prior promise must all be answered at once.