"< Sister·Lover > Japan's Latest Sibling Relationships Online"

Rai (played by Jun Matsumoto) and Iku (played by Nana Eikura) are twin siblings, both in their third year of the same high school. Rai is a well-known scholar at school, excelling in sports as well, with a cool personality and handsome looks that make him very popular with girls. On the other hand, Iku is quite the opposite—average grades and clumsy. The siblings had a very close relationship when they were younger, even making the innocent promise to marry each other and stay together forever in the future. However, recently, Iku has noticed Rai becoming increasingly distant towards her, while Yano (played by Yuta Hiraga), a childhood friend, suddenly confesses his love for her, further confusing Iku.

One night, Iku wakes up startled by unusual sounds and discovers that Rai, who should have been sleeping on the top bunk, is about to kiss her. Rai tells Iku that he has always loved her—a love that transcends sibling affection. Iku responds to Rai's feelings with a kiss of her own initiative.

Iku's friend Yuka (played by Ayaka Komatsu) discovers the special relationship between the siblings and threatens to reveal their secret unless Rai becomes her boyfriend. Meanwhile, their observant mother (played by Yoko Asano) also begins to notice something unusual about them. Thus, Rai and Iku, who once firmly believed they would love each other forever, face an unexpected test...