Sugisaki Ken, through diligence and academic excellence, had successfully entered the paradise that is the Hekiyou Private Academy's Student Council. There, he boldly embarks on his plan to create his personal harem with the 4 girls who are the other council members, namely: The incredibly youthful president Sakurano Kurimu; The cool and kindly yet super-sadistic secretary Akaba Chizuru; Tomboyish and hot-blooded vice-president Shiina Minatsu; and The ephemerally beautiful yet complicated treasurer Shiina Mafuyu. While every day since then has been non-stop fun, Graduation Day now looms near. Could this spell the end of their carefree days? (Source: translated and adapted from official site by Cranston)

Sakuta Azusagawa's little sister, Kaede, is wrapping up middle school and needs to make an important decision. The school counselor believes it is in Kaede's best interest to attend an online high school, given her history of being a shut-in and her crippling phobia of traditional classrooms. However, Kaede wishes to enroll in Minegahara High School, just like her brother, as she desperately hopes for a typical high school experience. She pleads with Sakuta; his girlfriend, Mai Sakurajima; and Mai's half-sister, Nodoka Toyohama, to help her study for the entrance exam—and they enthusiastically agree. But her envisioned high school life does not unfold as planned. With her unresolved trauma coming to a head, Kaede grapples with the conflict of determining what sort of future path she truly wants to embark upon.

Ousai Academy was originally an all-girls high school. Due to the declining birth rates in recent years, it was converted into a co-ed school. The first year male students would find themselves surrounded by girls and unfortunately, Tsuda Takatoshi is one of them. What's worse, he gets scolded on his first day by the student council president Shino Amakusa, which did not give a good first impression of him. Tsuda also meets the other student council members while getting scolded, and in the end, he gets late for class. As an apology for ruining his morning, Shino lets him join the student council for various of reasons and he accepts it, or rather, he's forced to accept. Thus begins his days as Tsuda soon realizes that he's the only normal student in Ousai Academy...

For high school students like Shouichi Kamita, university entrance exams and the future are common concerns. It is also during this time in life that a mysterious emotion that vexes people of all ages may begin to weigh upon one's mind—love. At this point in his teenage years, Shouichi finds three girls he could see himself having a future with: the ever-cheerful Hikari Tsuneki, who teases Shouichi without mercy, but actually has a softer side; the competitive gamer, Tooru Miyamae, who has difficulty communicating with others; and Shouichi's childhood friend, Kyouko Touno, the sometimes immature shoujo manga enthusiast. Seiren follows Shouichi's relationships with these three girls across three separate arcs, as their feelings grow from mutual interest and blossom into something more beautiful.