The earliest surviving version of Lludd and Llefelys is preserved with the Welsh Brut tradition in the thirteenth-century Llanstephan 1 manuscript, while later witnesses occur in the Red Book of Hergest and fragmentarily in the White Book of Rhydderch. The tale itself calls Lludd and Llefelys sons of Beli Mawr ap Manogan. Lludd inherits Britain, rules prosperously and is said to rebuild the walls of London, which the story calls Caer Lludd. Those London details show knowledge of the British chronicle tradition, while other elements of the tale, especially the Coraniaid and the buried dragons, are generally regarded as preserving older Welsh material.

The first plague is the Coraniaid. Their extraordinary knowledge makes ordinary resistance impossible because any conversation touched by the wind becomes known to them. Lludd therefore sails to consult Llefelys in secrecy. The brothers meet at sea and try to speak through a long bronze horn so their words will not reach the wind. A demon inside the horn twists their speech into hostile words until Llefelys washes it out with wine. Once they can speak safely, Llefelys explains how the Coraniaid can be destroyed with water in which special insects have been crushed.

Lludd returns to Britain, summons his own people and the Coraniaid to a supposed peace conference and casts the prepared water over everyone. The mixture kills the Coraniaid while leaving the Britons unharmed. The Welsh Triads also remember the Coraniaid as the first of the Three Oppressions, although the earliest surviving triadic text names Caswallon son of Beli where later discussion connects the episode with Lludd. The variation is a useful reminder that the medieval Welsh traditions did not always preserve one perfectly fixed genealogy.

The second plague is a scream heard over every hearth on May Eve. Llefelys explains that two dragons are fighting, one associated with Lludd's people and one with a foreign people. Lludd measures Britain and finds its centre at Oxford, where he places a cauldron of mead beneath a silk covering. The fighting creatures eventually fall in the form of two pigs, drink the mead and sleep. Lludd wraps them in the covering and buries them in a stone coffer at Dinas Emrys in Eryri. A later Welsh triad treats the burial as one of Britain's great concealments. The famous red and white colour symbolism belongs to the later dragon story associated with Vortigern and should not be read back into Lludd and Llefelys itself.

The final plague is the disappearance of the king's food and drink. Llefelys identifies the thief as a powerful magician who sends everyone to sleep through enchantment. Lludd keeps himself awake with cold water, catches a huge armed man carrying the provisions away in an enormous hamper and defeats him in combat. The defeated thief promises restitution, swears never to repeat the offence and becomes Lludd's loyal man. The tale closes with Lludd ruling Britain in peace after all three plagues have ended.

Lludd ap Beli is often combined in modern mythography with Lludd Llaw Eraint, the Silver-Handed Lludd named as Creiddylad's father in Culhwch and Olwen, and with Nudd, Nuada and the Romano-British god Nodens. That comparison has an important history in Celtic scholarship, but it should not be converted into a certain medieval family tree. P. C. Bartrum treats Lludd ap Beli and Lludd Llaw Eraint separately and notes that Welsh literature itself does not establish the proposed Lludd-Nudd identification. For Mythopouch, this record therefore concerns the king in Lludd and Llefelys rather than the separate Silver-Hand figure.