(While I know this will be very familiar in some circles, it is clearly not known to the population at large. It ought to be. Naturally this is just one version out of many.)
A handsome young man was being held captive by the fairy folk. Tam Lin had been a favorite of the Queen’s for some time but she had finally found a new toy, and Tam Lin was to be the fairies’ next human sacrifice.
He met in secret with his lady, Byrd Janet, and told her what would happen, begging her to be brave enough to rescue him from his fate. Giving her detailed instructions about how to identify him among the crowd, he explained what would happen.
On All Hallows Eve Byrd Janet was to make a circle of holy water to stand in for protection, then watch the procession of the fairy folk. She must let the riders of the black horse and the brown horse pass by. Tam Lin would be riding the third– a white horse. Janet was to run to him, pull him down from the horse, and hold him; no matter what might happen.
He warned her the fairies would change his shape in her arms, but she must never let go, until she could render him human again.
It all happened as he had described.
When Byrd Janet pulled Tan Lin from his horse the entire procession halted. The folk gathered all about, trying their magic on Tam Lin. They turned him into freezing ice, a poisonous serpent, and a struggling dove who almost escaped.
Tam Lin was brought through a surging struggle of transformations until the fairies turned him into a piece of red-hot iron.
With this in her arms, Byrd Janet rushed to a nearby well and cast it in, revealing her beloved, naked, in his true form. She threw her green mantle around him, covering his nakedness and claiming him.
At this there was a great grief and wailing among the fairy folk, and their Queen declaimed in verse that she would have blinded Tam Lin, or exchanged his heart of flesh for a heart of stone, rather than lose so fair a knight to a mortal girl.