Welcome to Twenty, a wild partner-swap reality show for four teen couples on the edge of adulthood. Secret billionaire heiress Han-byul sacrificed everything for her boyfriend, only to be gaslighted and used as his personal ATM. But the game changes when a massive wild card enters: Sun-woo, a superstar swimmer. Suffering from trauma-induced mutism, the brooding athlete has shut everyone out—except Han-byul. Can she finally cut her toxic ex loose and dive into a new romance?