Fate/Zero Season 2 - Distant Memories

Years ago, a young Kiritsugu lived on an island in the Pacific with his father Noritaka and befriended a local girl named Shirley who called him Kerry. Noritaka was researching on immortality using plants with Shirley as his assistant. Despite being told by the townsfolk to stay away from Noritaka, Shirley is convinced his research will help the world and tells Kiritsugu she hopes he grows up to be great man. But one day, Kiritsugu is told by his father not to leave their home after asking him if he went into his lab. Kiritsugu ignores his father’s warnings to search for Shirley only to be find her eating live chickens wildly. Shirley reveals she went into Noritaka’s lab and use his immortality formula on herself which turned her into a Dead Apostle vampire who madly craves for blood. Shirley begs Kiritsugu to kill her while she can still have a hold on herself but he is unable to do it which leads her to attacking the townsfolk and turning them into Ghouls. As chaos spread throughout the island, Church Executors and Mages from the Mages Association arrive to stop and exterminate the outbreak and cover up the incident. Kiritsugu is saved from Ghouls when a gun-wielding mercenary named Natalia Kaminski saves him. Natalia explains she is there to eliminate the magus responsible for the outbreak which Kiritsugu realizes is his father. Kiritsugu confronts his father over his work and it’s consequences which the latter only shrugs it off as an accident and states his plans to continue his experiments elsewhere. Angered with his father’s response, Kiritsugu kills him. Emotionally broken and having nowhere to go, Kiritsugu agrees to escape together with Natalia.