July 1st, 2030. Manga editor Nishino Mizuho is having the worst 27th birthday ever. The manga she’s in charge of will be ending. However, when she sees a group of high school students having fun, she recalls her own adolescence. 10 years ago, during her second year of high school, Mizuho had four childhood friends who she grew up with in a residential neighborhood known as Koigahama Highlands…

Kizuki, who Mizuho saw as the crybaby little brother of the group, has lately been hitting on her. Since Mizuho believed her and Kizuki would be childhood friends forever, she is not sure how to approach this situation. He's on her mind to the point where she seems disoriented, and even says something that could potentially push Kizuki away.

Afraid of ruining their relationship, Mizuho tells herself not to fall for Kizuki. All the stress makes her ill, and her friends come to help.

After spending the day at the aquarium, Kizuki asks Mizuho to cheer for him at his swim meet.

As the fireworks are set off, Mizuho rushes to look for Kizuki. At the pool she runs into...?

Mizuho is reluctant to visit Kizuki's house for dinner while her dad is away.

Shugo's older brother, Togo, makes a surprise return to Koigahama Highlands.

Mizuho goes out with Shugo and Togo and strongly contemplates her feelings for Kizuki.