KissXSis

Keita Suminoe has a problem with his twin step-sisters, Ako and Riko, being overly romantic towards him. However, he wants to attend the same high school as them, so is uncertain on what to do when he has to attend a parent teacher session involving him turning down a recommendation. When Ako and Riko find his notice letter whilst looking in his room, they go over to his school, only to be chased by several of their old admirers. Keita stops everyone and tells his sisters off for bothering him, but when he realizes why they came, he catches up with them and apologizes.

Ako decides to help Keita study in order to improve his passing rate, though both have a little trouble staying focused on the matter at hand. Ako shows Keita her chest in order to encourage him to study, but this ends up being all Keita can think about in class. Ako decides to try an alternative method, writing the notes on her body. Despite embarrassing himself in school whilst distracted, he fares better on the tests. However, when Keita likens a kiss from Ako as a penalty game, Ako gets mad and corners him, eventually making him do a wrong answer so he would have to take the penalty game. They hint at going further, but are stopped by Riko, who had been learning how to prepare food for Keita, though it ends up tasting bad. Despite this, Keita manages to improve his grade.

Keita gets annoyed with being kissed by Ako and Riko him all the time and instigates a ban on kissing. Overhearing that Keita likes sweet things, Riko tries to make him some chocolate, but can only manage something horrific. After finding one of her father's dirty DVDs, she gives Keita some chocolate lipstick, persuading him to eat it off her lips. Before they manage to go any further, they are interrupted by Ako who destroys the lipstick. Riko gives Keita another chocolate lipstick, however it was made from the horrible tasting chocolate that she had made.

While Keita studies at school, Ako and Riko reminisce on when they started going to junior high, the first time they were apart from Keita. When they watched Keita perform athletics, they fell in love with him. Meanwhile, their teacher, Yūzuki Kiryū, eavesdrops on their discussions and begins to worry about what is going on with them. After studying, Keita runs into Ako and Riko at the shrine, where they were praying for his success. He remembers when he was young and prayed to marry both Ako and Riko, only to be told by his father that polygamy is illegal. Keita makes his wish and the three return together.

While talking to his friends, Keita accidentally gropes his fellow student, Miharu Mikuni, and gets slapped. Ako and Riko become suspicious about whether Keita has been 'cheating'. When Keita goes to study at the library, Miharu, who is the librarian, becomes too intimidated by Keita to go to the bathroom. Before she can go to one, Keita tries to return a book she dropped and chases after her, dragging her into various hiding spots to hide from Ako and Riko. However, they end up getting locked in the gym storage room and their hashed escape plan leads Miharu to relieve herself on Keita's back. They eventually manage to escape and Keita promises to keep Miharu's incident a secret, though gets beaten up when he accidentally sees where Miharu had removed her panties from.

Unable to view and get porn from the internet using a PC their father won at a raffle, Ako and Riko decides to buy erotic games in Akihabara. But in the process Keita has to accompany them, so they devise a way to hide their real purpose for going to Akihabara. Ako distracts Keita by taking him to a cosplay shop whilst Riko investigates the 18+ section of a game shop. Whilst changing, they run into Ako's teacher, Yūzuki, who is a closet otaku (dressed as Mahoro from Mahoromatic Maiden). Riko later encounters her whilst trying to get a special edition game. After meeting Keita and being reminded about Ako and Riko's crush on him, Yūzuki drags him off to talk to him, inadvertently taking him to a cosplay-themed love hotel. After later being found by Ako and Riko, the group are encountered by Keita's kōhai and Yūzuki's younger sister Mikazuki, who covers for Yūzuki by stating she was buying the game for her.