Lamune follows Kenji, a boy who has grown up with his childhood friend Nanami. Although everyone knows they share a deep bond, neither Kenji nor Nanami is ready to turn their friendship into something more. The series intersperses present‑day scenes with flashbacks that reveal how their friendship began and why it remains so strong, and even the most casual interactions only serve to deepen their connection. Later rebroadcasts added short scenes that were originally released only in the DVD volumes.