The members of TXT move into an old boarding house by the sea for a summer retreat, hoping to relax and focus on music. But strange things start happening after the first full moon. Taehyun begins having vivid dreams of a girl crying on the cliffs. Beomgyu hears whispers coming from the ocean. Soobin finds old, unfinished sheet music in the attic that only plays during moonlight.

Each night, as the moon stays full and the town refuses to sleep, the boys begin uncovering the tragic love story of two musicians from decades ago—one of whom vanished under mysterious circumstances. The closer they get to the truth, the more they find themselves tied to the past, their own emotions, and secrets they didn’t know they had.

But what happens when the moon finally starts to wane again?