Hiro’s childhood friend Chiwa is like a little sister to him. Instead of a promise to marry each other, they made a promise with a toy choker to keep a relation like a dog and its master. When Chiwa left with her mother to England, she vowed to return to Japan one day and be in the same class in school one day. A few years later, she finds out that her mother will be remarrying his father. However, if she becomes his true imouto, does that mean that she can’t marry him anymore!? She hurries back to Japan and asks him if he remembers their promise. She offers to become her oniichan’s pet and wants him to love her a lot.