Murasaki (protagonist) and Nodoka (heroine) were childhood friends who grew up next to each other. They were like brother and sister.

"I want it to be like this always."

But then Mr. Ryuji Oda changed everything.

Oda was a new teacher who appeared at school. In no time, he drew indecently close to Nodoka. His adult waft drifted between Nodoka and Murasaki, disrupting their relationship.

[From DLsite English]