So begins the lai of Sir Orfeo, the king who ventured into Faerie to rescue his wife. Told in manuscripts from the 14th century, the story makes use of the British idea of a fairy realm as a substitute for the afterlife (a folkloric conceit likely derived from lightly Christianized retellings of ancient Celtic myths) and a courtly setting in medieval Britain - but, ultimately, the tale of Orfeo and Heurodis is an adaptation of Greek works.
Sir Orfeo, distraught by the loss of his wife, secluded himself in the forest for a decade. Then he saw Heurodis ride past in the company of the King of Faerie, and was determined to rescue her and return to their palace. On entering the castle of the fairy king, intent on a rescue, Sir Orfeo saw the bodies of many sleeping people once thought dead. Skilled with a harp, he entertained the king, who offered him a chance to choose his own reward; Orfeo requested the return of his beloved wife.
The two returned, but, ragged and unkempt, were mistaken for beggars. In that guise, Sir Orfeo played his harp, and told his steward he took it from the king's corpse years before; seeing that the steward weeps for his lost lord, Orfeo revealed himself, saying that had the news brought joy he would have exiled the steward from his kingdom. Instead, he named him its heir.
The parallels to the classical myth from which the characters take their names - that of Orpheus and Eurydice - are obvious; Orfeo even plays the harp, like the lyre strongly associated with the Greek Orpheus. The ending, of course, differs - Heurodis is successfully rescued from the Otherworld, unlike Eurydice. (Even in the ending we can find a certain level of similarity to Odysseus's return to Ithaca and his challenging of his suitors, though the loyalty test motif is certainly bigger than just Homer's use of it and this similarity may be by chance.) Nonetheless, the setting of the story makes it clear that, while the anonymous author knows they are retelling a story from a classical source (Sir Orfeo rules from Thrace, though we are told this is a former name for Winchester, in England), the story belongs to the writer's own time. Its setting is the familiar courtly life of medieval England and the realm of Faerie as known in English lore. The narrator has no desire to tell an old Greek myth; rather, Sir Orfeo is a tale written to be understood in a contemporary context, even though it clearly takes place well in the past (when England was not a single kingdom and Winchester was known by another name).
It's likely the story had been transmitted in this form before being written down in rhymed stanzaic Middle English verse. If so, it's also probable that the altered setting - a time long ago, but recognizably like the teller's own present - is part of what helped the story survive to be retold in this new English form.
A similar alteration affects the narrative presentation of classical material throughout the Matter of Rome. Just as Alexander the Great is presented as a medieval king and the warriors at Troy as chivalric heroes, Sir Orfeo is an exemplary medieval ruler who shows honorable devotion to his lady, and whose story belongs entirely to those who tell it, rather than to the ancient people on whose myth it was based.