"The one in that house isn’t my sister." In the distant future, Kagamimi Village is a mountain village on the outskirts, left behind by the densely developed science and technology of urban areas. “If you’re not a good child, your double will come for you.” Akane Higurashi, a middle school student living in this village steeped in old legends, has been looking forward to reuniting with her older sister, Sui Higurashi, who has just been discharged from the hospital after a year. But soon after her sister’s return home, Akane receives a phone call. "The one in that house isn’t my sister." The “Oneechan Gokko” series is a long-form animated work and a series of VOCALOID songs, with each episode running about 25 minutes, for a planned total of 3 episodes. Starting in January 2026, the songs and each episode will be released one after another, with the goal of completing the project over the course of about one year.