Arisa and Mika Nanase are sisters and the school super-idols. They are the hottest and cutest girls in the school. Sousuke is a normal student who like the rest of the boys, can only look at the super-idol sisters from afar. Sousuke likes Arisa but can never hope to even speak with her. One day Mika tells him she want to speak with him after school. Later, he meets with Mika and Arisa and they reveal that they have now become siblings! The truth is that Sousuke's father, an adventurer, and the Nanase sister's mother, an archeologist, met at an excavating ruin in South America, fell in love, and married.
Mika, Arisa, and Sousuke are now siblings and will live under the same roof from now on. Sousuke now has to be careful to not get caught masturbating by Mika or Arisa ...or maybe they might just give him a helping hand... How about taking a bath with Mika? ...Siblings do bathe together after all...