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.

One of Keita's friends coaxes him into going to the pool with him and Mikazuki and inviting Ako and Riko. Suspicious of what they might be up to, Yūzuki decides to tail them. Whilst trying to hide from them in the bathroom, she is found by Ako and Riko who, mistaking her for a peeping tom, splash her with water. With her clothes soaked, Yūzuki borrows Keita's gym outfit. Later that night, Yūzuki becomes entranced by the smell of Keita's outfit and masturbates whilst wearing it and having sexual fantasies about Keita. She wakes up the next day to find that she had soaked his pants in the process.

The summer festival comes, and it will be the only day Keita will be free from all the prep school work. This means that Ako and Riko will be competing for a night at the festival with him, although they all go to the festival together. But a few incidents involving paraders enables Ako and Riko to indeed spend some time with Keita alone. They also had to contend with Kiryu-sensei (who was patrolling the fair and catches Riko and Keita in the forest), Mikazuki, who is Miss Mikoshi, and a (ahem, naughty) chick that Ako bought that caused Keita to see Miharu peeing in the bushes. Shocked, Miharu drops on the ground what seems to be a love letter.

As Keita tries to study, Ako and Riko compete with each other to be the most helpful. Whilst alone in the room, Riko and Keita end up doing some 'footplay' under the kotatsu until Ako catches on. Whilst looking up on good luck charms to help Keita with his studies, Ako and Riko learn of one that is made using pubic hairs. After much deliberation, the sisters pluck their own hairs to make the charm and give it to Keita, convincing him not to look inside. Keita takes it to school with him, though Ako and Riko are understandably embarrassed.

With Keita ill before the big exams Ako and Riko step up to the mark by getting out of school and making Keita feel better by nursing him back to health, but a slow recovery is the last thing Keita wants as his two stepsisters fight for the place closest to his heart. Both of them end up licking the sweat off his body, which passes the cold over to them whilst Keita makes a full recovery. The final exams are overseen by Yūzuki, who is surprised to find Keita there. Whilst Keita takes his exams, Ako and Riko remember how Keita was there for them when they were taking their exams. After returning from his exams, Keita gives his thanks to a sleeping Ako and Riko, who are surprised about it once he leaves.

Keita eagerly awaits the results of his exams, with Ako and Riko trying to give him good luck, though encounter several unlucky omens along the way. When they get to the notice boards, Keita's number wasn't listed and the trio become depressed. After noticing the celebratory preparations Ako and Riko prepared, Keita cheers up and decides its best not to waste it. Later that evening, Keita is visited by Miharu, whose dog had previously nabbed Keita's posted results from his mailbox. After reading them, Keita learns that he had in fact passed the exams, hugging Miharu in his excitement. After Miharu gets scared off by Ako and Riko, Keita tells them the good news.

After Keita's graduation, Ako and Riko arrive to find they were too late to get the second, or any button, off of Keita's blazer. They decide to, against Keita's will, hold a draw for consolation prizes from Keita, with them, Mikazuki and Miharu as participants. Ako's hope backfires and ends up getting Keita's shirt collar, Mikazuki wins the opportunity for Keita to flip her skirt, though he is unaware she wasn't wearing any underwear. Riko wins a chance to kiss Keita in front of everyone, but is too embarrassed to do it. Miharu gets the remaining prize, to spend ten minutes in the equipment room alone with Keita to do anything. Keita simply uses the time to thank Miharu, but after a slip up, they end up in a compromising position, leading Ako and Riko to assume the worst. Later that night whilst Keita goes to sleep, Ako and Riko make a move on him, though this just gives Keita weird dreams.