Manga editor Nishino Mizuho is having the worst 27th birthday ever; the manga she's in charge of will be ending; when she sees a group of high school students having fun, she recalls her own adolescence.

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.

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.