Kilhwch, Arthur's cousin, is born in the mountains to a mother, who becomes wild during her pregnancy. Shortly afterward she dies, and seven years later, her husband conquers King Doged's land, kills him, and takes his wife and daughter. The new queen speaks of a prophesy that says Kilhwich is destined to have Olwen, the daughter of Yspaddaden Penkawr.
Kilhwch journeys to Arthur, and tells him he want Olwen as wife. Not knowing who she is, Arthur sends messengers to seek her, but at the end of a year she is not found. Kai, one of Arthur's knights, requests him to go with a group to find the maiden. Arthur calls Bedwyr, Kynddelig the Guide, Gwalchmai, a nephew of Arthur, Menw, who casts spells, and others, to go, too.
Coming to a castle, Kai ask a herdsman whose castle it is, and is told that of Yspaddaden Penkawr. However, Kai and the other knights learn that no one who sought Olwen ever returned alive. They go to the herdsman's dwell. There they learn Olwen will be coming on her usual visit. The herdsman's wife tells then that Olwen made a pledge to her father not to go without his counsel, for he will die when she marries. The woman tells them to ask of her from her father, and perform what he requires to have her, else they might not escape with their life.
Going back to the castle, they kill the porters and enter. They meet Yspaddaden Penkawr, and tell them their mission. After some bickering and violence over three days, Penkawr imposes about 39 difficult tasks to prepare for the wedding feast. The most important task is to seize the comb and scissors from between the ears of the boar Twrch Trwyth. This is done during a chase that leads to the death of some of the men and the boar plunging into a river and then out to sea. (The comb and scissors should be regarded as treasure.)
At one point, Kai is angered by a ditty Arthur composed and no longer helps. The tasks accomplished, Kilhwch takes Olwen for wife, her father is slain, and his possessions taken.