In the Kingdom of Northumbria, a kind king takes a cruel witch as his queen after the death of his wife. The Kings son, Childe Wynd, has gone across the sea but his daughter is turned into a worm by the witch. The enchantment used is usually:
I weird ye to be a Laidly Worm, And borrowed shall ye never be, Until Childe Wynd, the King's own son Come to the Heugh and thrice kiss thee; Until the world comes to an end, Borrowed shall ye never be.
The spell can only be broken by a kiss from Childe Wynd, the prince who is away in a land across the sea. The prince returns and kisses the princess freeing her from the curse and the witch is turned into a toad.
The story has a lot in common with the Icelandic Hjálmþés saga ok Ölvis.
There is no authoritative version of the ballad. Robert Lambe "discovered' it as fragments, which makes sense if it had generated variants over the centuries since Duncan Frasier had originally penned it.
Robert Lambe was an expert on the origins and meanings of ancient obscure words, and helped track down the meanings of some of the words found in the ballads in the "Reliques".The Laidly Worm never made it into the "Reliques" but was reprinted in various other books since its discovery.
Lambe sent the fragments to his friend Bishop Percy, another antiquarian. Percy had embarked on a British Empire spanning project to collect all the oral and written lore and ballads, which he assembled into a volume called "Reliques of Ancient English Poetry"
After Richard the Lionheart was released in exchange for a hostage, the hostage took with him to Germany a copy of an Arthurian romance involving the Sovereignty of Ireland, a snake maiden. In the 1190's Zatikhoeven rewrote this tale as "Lanzelet" and renamed the Irish lady Elidia.
Assuming the Laidly Worm o' Spindleston Haugh is an authentic ballad written by Duncan Frasier, then Duncan Frasier may have heard "Lanzelet" or some daughter of the parent loathly lady narrative. Up until the mid 19th century scholars believed that the "Laidly Worm" was an authentic folk ballad from the past.