Every day, Y/N walks the halls of Seaside High like a normal teenager. Every night, they dive into the ocean and swim home to a mermaid kingdom. Their green tail grants them the rarest gift: the ability to shift freely between two worlds. But when two humans witness their transformation, everything shatters. Luke—a quiet, haunted transfer student—steps into the surf and awakens as a silver-tailed mermaid with scales that replay forgotten memories. He was once an exiled noble, erased from history, and now he's permanently trapped in the sea. Cass—Y/N's ruthless bully—dives into a pool and emerges as a crimson-tailed outcast with explosive power she can't control. She's also permanently aquatic, ripped from her human life forever. Now Y/N is the only one who can walk on land. The only one who can deliver messages, buy supplies, and keep their secrets. The only lifeline for a boy who can't remember his past and a girl who's lost her future. But Luke's memory-scales are humming with a dangerous truth: Y/N's own mother is the reason he was erased. And as the echoes surface, Y/N must choose—protect their family, or save the two people who now depend on them to survive.