Fate/kaleid liner Prisma☆Illya Movie: Vow in the Snow (or Oath Under Snow) is a prequel focusing on a young Shirou Emiya and his adopted sister Miyu in a world where magic is real, exploring how Shirou became a hero of justice; it details his life with Miyu, their search for her magical artifact, and the tragic origins of the Holy Grail War's influence on their lives, culminating in Shirou's resolve to save Miyu from being sacrificed for the Grail, even if it means becoming a "hero of justice".