Kim Sere - a pressured, high school girl who has spent her whole life living for her mother's pride, forced to be the top student in her class. While Ahn Keonho is the complete opposite - he is carefree, a swimming athlete, a model, can sing and dance, and raised only by his older sister. No pressure. No expectations. Just freedom. He is almost perfect. What happens when a blue sea leads them together?

Won Yerim has one goal. Debut and shine like the stars she's always admired. As one of the top female trainees at KOZ Entertainment, she's placed in an intense training program alongside with her fellow trainee girls-Minji, Hana, Rina, and Jisoo-will debut as AESTRA. Together, they chase a shared dream while navigating the pressure, competition, and silent sacrifices of idol life. Han Taesan, the cold and unreadable member of BOYNEXTDOOR, is also a trainee at KOZ and lives on the same floor of the company dorms-just rooms apart. He has a quiet personality-Yerim's opposite. But, he finds himself quietly admiring her passion, energy, and determination. His world, usually too quiet and controlled, feels strangely empty whenever she's not around. As AESTRA and BOYNEXTDOOR prepare to debut, both groups face inner conflicts, school pressures, and rising public scrutiny.

A female student (Cha Soo Bin / oc) investigate the mystery of how Class 2-3 vanished. (Kyung-jun x oc)