Rhiannon duly reins in her horse and waits for Pwyll to catch up.  She explains to him that she is Rhiannon, daughter of Hefeydd the Old, and that she has been betrothed to a man against her will.  The man whom she truly loves, she confesses, is Pwyll himself.  If Pwyll will consent to marry her, she is willing to defy her father and meet Pwyll in a year and a day's time, so that they may be married.

Pwyll, astonished, agrees immediately, and after a year and a day, their wedding takes place.  Pwyll overflows with joy at his wedding feast, and perhaps drinks a bit too much.  An auburn-haired stranger takes advantage of this by begging a boon from Pwyll, and Pwyll, feeling magnanimous, tells the unknown fellow to name his boon.

The stranger turns out to be Gwawl ap Clud (Light, son of the river Clyde), the very man to whom Rhiannon had been betrothed before marrying Pwyll.  He gleefully asks that Rhiannon be given to him in marriage in a year's time. 

"Never did a man make worse use of his wits than you have done," remarks Rhiannon observing Pwyll's mistake.  Pwyll, bound by his word as a ruler, cannot withdraw his boon, but Rhiannon reassures him that she has a plan.

A year later a wedding feast is prepared for Rhiannon and Gwawl.  Rhiannon endures a seat by Gwawl's side, waiting for Pwyll to fulfill his part of her plan.  Pwyll enters the great hall disguised as a beggar, and begs a boon of Gwawl.  "What might that be?"  Gwawl asks.

"I am poor,"  Pwyll explains.  "What I ask is for my bag to be filled with food."

Gwawl cannot see any reason why not, and orders his men to fill the bag with food.  The bag is an enchanted bag prepared by Rhiannon, and no matter how much food goes into it, it never becomes full.  Gwawl, perplexed, asks the beggar why this is.  "Oh, my bag is enchanted,"  Pwyll responds.  "It will only become full after a royal man treads upon the food with his feet."

"Do as the man asks,"  Rhiannon advises Gwawl.

"Very well,"  Gwawl agrees, and puts both feet into the bag.

Pwyll swiftly encloses Gwawl in the bag, and summons his followers to kick the closed bag and hit it with clubs.  Rhiannon's father, Heyfydd the Old, intervenes on Gwawl's behalf and prevents Pwyll from killing him.  Forced to relinquish his claim to Rhiannon, and to give sureties against returning for revenge, Gwawl returns to his own country to heal his wounds. 

Rhiannon's marriage to Pwyll, though, will not be an easy one.  After three years, the people of Dyfed become impatient, since Rhiannon has borne no children.  They petition Pwyll to find another wife, but Pwyll asks for a year's delay.

Before the end of the year, Rhiannon gives birth to a son.  The serving-women who watch over the newborn boy fall asleep, and when they awake, the child is gone.  Fearful of the wrath of Rhiannon and Pwyll, they decide to create a ruse to save themselves:  killing a litter of puppies, they smear blood on Rhiannon's hands and face, and leave torn flesh and bones by her side.  Raising a loud outcry, they awaken Rhiannon and accuse her of eating her son.

Rhiannon protests that she has done nothing of the sort, but the court Druids, unable to tell the bones of a puppy from those of a child, condemn Rhiannon for murder.  Pwyll still refuses to send her away, but she is made to do penance for seven years.  She must sit by a horse-block outside the Arberth gate and offer to carry all visitors up to court on her back.   Fortunately, very few such visitors ever agree to let her carry them.

Elsewhere in the Mabinogion, Rhiannon is described as the mistress of three marvellous birds, whose song can wake the dead and lull the living to sleep.