noun
side story; spin-off story
A narrative that runs parallel to a main story, often focusing on a secondary character or event. Common in discussions of novels, films, games, and manga.
この小説のサイドストーリーでは、脇役の過去が描かれている。
This novel's side story depicts the past of a supporting character.
ゲームのサイドストーリーを全部クリアするのに時間がかかった。
It took time to clear all the side stories in the game.
Borrowed from English 'side story'.