Akiko moves into a school dormitory with her brother Akito, when he transfers to her school. Owing to her brother complex, Akiko has a crush on Akito. With both of them in student government, will Akiko be able to share him with the other council members?

Akito catches Akiko reading a trashy romance novel. Akito, having trouble settling into his class, is given firsthand lessons in the concept of "tsundere"from both Akiko and Anastasia. Akito and his longtime friend Ginbe get into an argument.

Akiko asks Akito for help changing her light. Akito has his hands full maintaining the dorm, from fixing the plumbing to repairing the doors. The girls have differing opinions over allowing Akito to take in the laundry. Akito helps Anastasia settle in.

Akito brings home a special dessert, just for Ginbe to enjoy. Akito continues to fix up Anastasia's room, and the two of them discuss her hobby of collecting stuffed animals. Arashi has the girls take a bath together, to help them bond together better.

In a student government meeting, the council members discuss what they are going to call Akito. Kaoruko proposes that Akito write a short story to be published in a women's magazine. She grows concerned for him when she learns his sibling is a sister.

Kaoruko reads the latest installment of Akito's love story. She decides to stop by the dorm, where she meets the others, and asks them whether they think Akito has a sister complex. The council decides to separate Akito and Akiko into separate rooms.

Akiko is upset over not being consulted before Arisa, who claims to be Akito's fiancée, was hired as dorm manager. Later that day, Arashi proposes that Akiko and Arisa compete in a contest, to see which one of them is the better little sister to Akito.

The girls all go to find new swimsuits even though summer is still in the distance. They try frilly ones, sexy ones, and cute ones. Then, Arashi proposes a contest to show of their new goods and Akito must decide which one impresses him the most.

Akito follows Gin to the back of the school building to find out why she is acting so suspiciously. When Akito suddenly falls sick, everyone fights over who will nurse him back to health. They decide to take turns treating him how they see fit.

During her turn nursing, Ginbe reminisces about Akito's transfer to her school and how they first met. After Akito recovers he has to choose the winner of the contest he had no idea about. What is the prize and what will the winner do with such a chance?!

Anastasia can't find her underwear and is forced to spend the rest of the day without it, which proves troublesome when Arisa reports something strange in the woods and the gang decides to investigate. When their search turns up nothing and they lose each other whilst going home, Anastasia reminisces about how she and the Himenokoji twins met. Kaoruko has a fever-induced dream that she and Akito's cohabitants have entered a polygamous marriage with him.

Everyone gets separated and Akiko is left alone with Nasuhara. While contemplating her panty dilemma, Nasuhara reminisces about her first meeting with Akiko. What will happen to Akito and his crew of crazy girls that sleep under the same roof?