Pwyll is a prince of Dyfed, an ancient kingdom in southwest Wales. One day, he was separated from his friends during a hunting trip and stumbles upon a pack of hounds feeding on a slain stag. Pwyll drives the hounds away and sets his own hounds to feast, earning the anger of Arawn, lord of the otherworldly kingdom of Annwn. In recompense, Pwyll agrees to taking on Arawn's appearance and trade places with him for a year and a day, and takes his place at Arawn's court. At the end of the year, Pwyll engages in single combat against Hafgan, Arawn's rival, and mortally wounds him with one blow, earning Arawn overlordship of all of Annwn. After Hafgan's death, Pwyll and Arawn meet once again, revert to their old appearance and return to their respective courts. They become lasting friends because Pwyll slept chastely with Arawn's wife for the duration of the year. As a result of Pwyll's successful ruling of Annwn, he earns the title Pwyll Pen Annwfn; "Pwyll, head of Annwn".
Some time later, Pwyll and his noblemen ascend the mound of Gorsedd Arberth and witness the arrival of Rhiannon, appearing to them as a beautiful woman dressed in gold silk brocade and riding a shining white horse. Pwyll sends his best horsemen after her, but she always remains ahead of them, though her horse never does more than amble. After three days, Pwyll, himself, rides out to meet her and when he cannot catch her, he calls out to her in desperation. Only then does Rhiannon stop. She tells him her name and that she has come seeking him because she would rather marry him than her fiancé, the evil king Gwawl ap Clud. She tells him to come to her kingdom one year from that day, with his soldiers, and they will marry. A year after their meeting, Pwyll arrives as promised but accidentally and foolishly promises his beloved Rhiannon to Gwawl (her previous fiance). This occurs when Gwawl enters the court extremely distraught and sues for a favor of the king. Gwawl plays to the nobility and generosity of Pwyll, as well as his rashness and passion, and Pwyll tells Gwawl that whatever it is that he should ask him, that he (Pwyll) would give it to him. Gwawl of course asks for his fiancée, Rhiannon, which Pwyll, due to his naive promise, could not refuse. It is decided that they should all come back to the kingdom in one year's time for yet another wedding. (Pwyll and Rhiannon were not yet married the first time, the festivities had simply begun but no marriage ceremony had occurred.) Rhiannon devises a plan by which Pwyll might win her back from Gwawl. Pwyll enters the festivities of Gwawl and Rhiannon's wedding dressed as a beggar and asks Gwawl for a sack full of food. Gwawl nobly consents, not knowing the nature of Pwyll's sack. No matter how much food he took, the sack is never fully filled. Eventually, Gwawl became suspicious and asked to inspect the sack. The moment he looked inside the sack, Pwyll pushed him in and trapped him inside. Pwyll then began brutally beating him until he relents, allowing Pwyll to take back Rhiannon in exchange for sparing Gwawl.
A few years later, the nobles of the kingdom were worried that there would not be an heir to follow Pwyll and advised him to take a second wife. He set a date where this would happen, but before long Rhiannon gave birth to a boy. On the night of his birth, the boy was lost while under the care of six of Rhiannon’s ladies-in-waiting. Scared of the punishment, the ladies smear dog's blood onto a sleeping Rhiannon, claiming that she had committed infanticide and cannibalism through eating her child. The kingdom was horrified to hear this news. They demanded Pwyll to divorce and execute his wife for these charges. Pwyll refuses, believing in his wife's innocence. He tried to persuade the nobles to call off Rhiannon's punishment, but at most he was only able of changing the sentence from death to exile, forcing Rhiannon to stay outside the kingdom gates and tell everyone her story for the next seven years. During this time, Pwyll relentlessly looked for leads to prove her innocence.
The answer would only come at the end of those seven years. One day, Pwyll learned of a certain incident that took place on a nearby village around the time of his son's disappearance. A local lord named Teyrnon had a mare which produced a foal every year but would disappear the night it was born. To solve this problem, he brought the mare into his house and kept watch over it throughout the night. As soon as the colt was born, a great claw came through the window and grabbed the colt. Teyrnon cut off the arm and kept the colt but heard a roar outside and found a baby boy. He and his wife claimed the boy as their own and named him Gwri Wallt Euryn (Gwri of the Golden hair), for "all the hair on his head was as yellow as gold". The child grew to adulthood at a superhuman pace and, as he matured, his likeness to Pwyll grew more obvious and, eventually, Teyrnon realised Gwri's true identity. The boy was eventually reunited with Pwyll and Rhiannon and was renamed Pryderi, meaning "anxiety" or "care".
Pwyll's tale, the first branch of the Mabinogion, ended with Pryderi's ascension to the throne, succeeding his father. Pwyll himself lived to a ripe old age and died peacefully, closing the tale of his outrageous adventures.