Rino has a cousin, Mirai, who is much older than her. Rino has loved her like an older sister since she was a child, and before she knows it, she has special feelings for her. Mirai sometimes visits Rino's house, and she comes to see Rino under the pretext that she likes Rino and is coming to eat at her aunt's house. Mirai opens up Rino's Kokoro . And Rino steps into a forbidden world.