A young lord is taken prisoner by a foreign king. The king's daughter frees him after receiving a promise that he will wed her in seven years. Seven years later she comes to England to see him being married. When he sees her, he marries her instead
This song is commonly connected with the story of Gilbert Becket, the father of Thomas (the clerical adversary of England's Henry II). But, although the song's widespread currency implies that it is old, it is unlikely that it is that old.
Child believed that it may have been affected by the Becket legend, but was probably independent.
The plot very much resembles "The Turkish Lady" [Laws O26], and some scholars lump them, but the latter emphasizes the conversion of the princess rather than, as in this song, her pursuit and reunion.
It is interesting to note that, according to the tranlation of _Sir Gawain and the Green Knight_ made by James L. Rosenberg and edited with introduction by James L. Kreuzer (Holt, Rinehart, and Winston, 1959, p. LII, the "love interests [in the crusading romances] were frequently unintentionally humorous: the Saracen princesses in literature were almost as aggressive in their behavior as the knights they aided."
A minor footnote: In the Scottish "Young Beichan" texts, the Turkish girl is typically called "Susan (Susie) Pye," with no obvious derivation that I can see. In the more numerous "Lord Bateman" texts, however, she is usually "Sophia." But "Sophia" (Greek for "wisdom") is not a Turkish name. Perhaps the girl had more reasons than love for wanting to escape. One might even speculate that she had (or that some singer intended her to have) a Christian mother. Or that she would rather marry an infidel than live in a harem.
And, yes, that's an awfully long chain of inference to hang on one name.... - RBW