The tale's anonymous Welsh author draws together family drama, courtly display and marvels. Its composition is often placed around 1100, but proposed dates and the relationship between written and oral material remain debated. A version survives in the White Book of Rhydderch, copied in the middle of the fourteenth century, and a fuller text in the Red Book of Hergest, copied around the end of that century. These surviving manuscripts are later than the proposed composition. They preserve an important literary work without supplying an exact date for every element of its story.

Culhwch is the son of Cilydd and Goleuddydd. His unusual birth among pigs supplies the tale's explanation of his name. After Goleuddydd's death, Cilydd eventually remarries by killing King Doged and taking his widow. The new queen proposes that Culhwch marry her daughter. When he refuses, she places a condition upon him: he may marry no woman except Olwen. The description of Olwen awakens his love before he has seen her. Cilydd advises him to seek Arthur, turning a household conflict into a journey towards the heroic court.

At the entrance Culhwch encounters the formidable porter Glewlwyd Gafaelfawr. Once admitted, he seeks Arthur's help and invokes an extensive list of members of the household. Warriors, women and figures with remarkable powers give the court its distinctive character. Familiar names appear in Welsh forms, including Cei, Bedwyr and Gwalchmai, whose later counterparts include Kay, Bedivere and Gawain. The catalogue is part of Culhwch's appeal for his boon, not simply Arthur swearing an oath over a roll call. Arthur's resources then become available to the quest.

Ysbaddaden sets a chain of difficult requirements for the marriage. Among them are animals, implements and people needed for his preparations, including objects carried between Twrch Trwyth's ears. The search for Mabon leads Arthur's companions through a succession of ancient animals to the Salmon of Llyn Llyw. Mabon had been taken from his mother when he was three nights old; the tale does not say he had been imprisoned since the beginning of the world. His rescue at Gloucester enables him to take part in the later hunt.

The pursuit of Twrch Trwyth carries the company across Ireland and Britain and costs the lives of several companions. A comb, razor and scissors must be recovered, rather than merely killing the boar. The surviving narrative gives particular adventures extended attention instead of describing every demand at equal length. At the conclusion Caw shaves Ysbaddaden, and Goreu then beheads him; the shaving and killing are separate actions. Culhwch marries Olwen. The tale's enduring force lies in the scale of the assistance gathered around a personal request: a family difficulty draws an entire world of extraordinary companions into motion.