prequel
noun [ C usually singular ] uk/ˈpriː.kwəl/ us/ˈpriː.kwəl/
（据已问世文艺作品的情节凭想象上溯创作的）先行篇，上篇，前传
a film, book, or play that develops the story of an earlier film, etc. by telling you what happened before the events in the first film, etc.
Jean Rhys's novel "Wide Sargasso Sea" is a prequel to Charlotte Bronte's "Jane Eyre". 琼•里斯的小说《藻海无边》是夏洛蒂•勃朗特的《简•爱》的前篇。
Compare: sequel