A girl waiting for her sister to come home discovers something wearing her face instead. The folklore about Doubles stops being a bedtime threat and becomes the thing standing in your kitchen. Playing Sister knows exactly how to weaponize the uncanny distance between "this looks like her" and "this is not her," and it does not blink first.

In the distant future, science and technology have become concentrated in the cities, and remote places like the mountain settlement of Kagami Village have been left to fall behind. Locals still repeat an old saying that children who misbehave will be visited by a Double. Middle schooler Akane Higure has been eagerly counting the days until her older sister Sui comes home after a year in the hospital. Soon after Sui returns, Akane gets a phone call claiming that the girl in the house with her is not really her sister. The three-episode ONA blends mystery, psychological thriller and supernatural elements.