The song tells the tale of a young man, who tells the listener to ask his former lover to perform for him a series of impossible tasks, such as making him a shirt without a seam and then washing it in a dry well, adding that if she completes these tasks he will take her back. Often the song is sung as a duet, with the woman then giving her lover a series of equally impossible tasks, promising to give him his seamless shirt once he has finished.
As the versions of the ballad known under the title "Scarborough Fair" are usually limited to the exchange of these impossible tasks, many suggestions concerning the plot have been proposed, including the hypothesis that it is a song about the Plague. In fact, "Scarborough Fair" appears to derive from an older (and now obscure) Scottish ballad, The Elfin Knight (Child Ballad #2), which has been traced at least as far back as 1670 and may well be earlier. In this ballad, an elf threatens to abduct a young woman to be his lover unless she can perform an impossible task ("For thou must shape a sark to me / Without any cut or heme, quoth he"); she responds with a list of tasks that he must first perform ("I have an aiker of good ley-land / Which lyeth low by yon sea-strand").
As the song spread, it was adapted, modified, and rewritten to the point that dozens of versions existed by the end of the 18th century, although only a few are typically sung nowadays. The references to "Scarborough Fair" and the refrain "parsley, sage, rosemary and thyme" date to nineteenth century versions, and the refrain may have been borrowed from the ballad Riddles Wisely Expounded, (Child Ballad #1), which has a similar plot.
Much thought has gone into attempts to explain the refrain "parsley, sage, rosemary and thyme", although, as this is found only in relatively recent versions, there may not be much to explain. The oldest versions of "The Elfin Knight" (circa 1650) contain the refrain "my plaid away, my plaid away, the wind shall not blow my plaid away" (or variations thereof), which may reflect the original emphasis on the lady's chastity. Slightly younger versions often contain one of a group of related refrains:
- Sober and grave grows merry in time
- Every rose grows merry with time
- There's never a rose grows fairer with time
These are usually paired with "Once she was a true love of mine" or some variant. "Parsley, sage, rosemary and thyme" may simply be an alternate rhyming refrain to the original. Folksong scholar Märta Ramsten states that folksong refrains containing enumerations of herbs — spices and medical herbs — occur in many languages, including Swedish, Danish, German, and English.
Parsley, sage, rosemary and thyme may also refer to the pagan belief, that when together, can be a love charm.