prequel

esiosa, etko-osa puhekieltä In a series of works, an installment that is set chronologically before its predecessor, especially the original narrative or (perhaps improper usage) any narrative work with at least one sequel.
1980, Patrick Robertson, Movie Facts and Feats: A Guinness Record Book, New York: Sterling Publishing, p. 43:

'Prequels' are sequels that relate the story that preceded the original film.

They are also threatening to block the production of the long-awaited prequel, The Hobbit, which may now be cancelled.