Sayuri-san no Imouto wa Tenshi revolves around siblings Sayuri and Mikoto Kobayashi, who were separated for 13 years after their parents' divorce. Mikoto surprises Sayuri by showing up at her flower shop one day with wings and a halo, which she claims were bestowed upon her by God after she prayed to be reunited with her sister. As a result, she moves in with Sayuri.