Keita Suminoe has trouble resisting the sexual advances of his twin stepsisters, Ako and Riko.

Keita injures his hand at school and Ako and Riko compete to help him out.

At Christmas, Keita, Ako and Riko go to a karaoke bar where they all end up getting very drunk.

Whilst searching through Keita's room, Ako and Riko find a dirty magazine featuring siblings in a threesome. Wondering if they may end up doing the same thing with Keita, they end up having a "practice threesome" with a dummy.

Keita, Ako, and Riko go on a trip to a Hot Spring together.

While studying together, Keita, Ako, and Riko try to block out the sounds of ecstasy coming from their parents' bedroom, and end up in a compromising position in the process.

As the school takes a fitness exam, Ako and Riko compete with each other for the best results. Meanwhile, Keita gets into all sorts of trouble trying to keep the other boys from spying on the girls.

As Keita gets soaked on a rainy day, Mikazuki invites him to her house, where he ends up having to hide from Yūzuki when she suddenly comes to visit.

Yuuzuki visits Keita's home for a school visit, where their luck runs true to form and something embarrassing happens. Fortunately for Yūzuki both of Keita's parents are very open minded - maybe a little too open minded.

Whilst Riko is away, Keita develops a taste for licking Ako's sweat. Later that night, Riko makes her own moves on Keita, only to also end up on the receiving end of his newly awakened fetish.

Keita inadvertently steps into the girl's bath at the public bathhouse and is forced to hide when Ako, Riko, and the others show up.

Keita, Ako, and Riko play a game in which they exchange bits of tapioca between their mouthes. After realising that the tapioca juice was actually alcoholic, Ako takes advantage of Keita's drunkness, soon discovering that he possesses memories of the previous times he was intoxicated. Riko attempts to do the same, only to have it backfire on her.