College student Kaho has been smitten with up-and-coming novelist Tomoki, who is also her adoptive brother, for as long as she can remember. But, not wanting to ruin her existing relationship with him, she kept her feelings hidden and moved out to live alone. However, due to an unexpected mishap, she ends up moving back in with Tomoki at home - and realizes being at his side as a "sister" isn't enough. Kaho has just reaffirmed her love, but then it turns out another person is already staying at the house! And not only does this Atsufumi notice how she feels about Tomoki, but his condition to keep her secret is: "be my girlfriend"?! Adoptive siblings + the brother's friend - it's an evergrowing chain of unrequited love in this housemate love triangle!