Lamune tells the story of a boy named Kenji and his childhood friend Nanami. Although, everyone seems to know that they have a relationship, neither are determined to push it anymore than close friends. The story goes through flashbacks, explaining why they are such close friends and any action doesn't seem to push them apart, but draw them closer.

Himari Kino is a cheerful and outgoing girl who is able to make friends with anyone she wants. When she enrolls in high school and attends the freshman opening ceremony, Himari falls in love with Yori Asanagi, a reserved upperclassman, during Yori's band performance. After the show, Himari blurts out her love, but Yori is inexperienced when it comes to romance. Yori, who did not expect this outcome at all, looks for answers from her friends and music. She decides to respond to Himari's confession, and to her surprise, Yori discovers that Himari fell in love with her music and performance rather than Yori as a person. Despite the shock, Yori desires to make Himari truly fall in love with her. The duo spend time together after school on the rooftop, where Yori usually plays her guitar with hopes to change Himari's feelings for her.

Tomoya Mikami's life changed one day when his girlfriend Ayaka was hit by a car and killed on her way to meet Tomoya. He blamed himself for her death and never was the same after the accident. He began acting callously towards Yue and his other friends. Furthermore, every time Tomoya gets involved with a new girl, he sees images of Ayaka in his dreams. He decides he needs to find a way to say goodbye to his lost love once and for all, while dealing with several side romances at the same time.

Shirakawa Hotaru, a girl who's in love with Inami Ken, finally confesses her love. From there on, they were happy couples, until one summer. Ken met a new friend, Tobise Tomoe, who is Hotaru's best friend. Tomoe seems to have her own charm. Ken, now have two paths to choose from; to be with Shirakawa Hotaru or Tobise Tomoe. Otherwise, he will lose them both.