Iroha, a high school girl who lives with her mother, has a sister called Rika who is close to her age due to her mother's remarriage. Iroha hits it off with Rika, who has a straightforward personality, probably because she lived with her father, and she also knows that Rika has been a lesbian with a girlfriend and a sex friend since she realized she was a lesbian when she was a high school girl. Rika has been secretly eyeing Iroha, and finally...!