We're J-Just Childhood Friends

Yuuichi Momose and Sumi Fujino are childhood friends who have been together since they were little. Now, they're high school classmates, but since they've reached puberty, they don't feel as close as they used to. They know each other well, but there's a sense of formality there. This is the delicate relationship between them, until one day, Sumi suddenly comes to Yuuichi's house...?