The reality show <Twenty> features four couples on the cusp of turning twenty, as they swap partners! Wang Han-byul, the heiress of King's Group, gave everything to her boyfriend Kang Jun-hwan, only to be met with betrayal and gaslighting. Han-byul even gave up her own identity to Jun-hwan, reducing herself to a mere wallet for him. Then, an unexpected variable appears before Han-byul: Ji Sun-woo, the nation's first love and a star swimmer! Sun-woo, who hadn't opened his heart to anyone since developing selective mutism after an accident, begins to feel special emotions only for Han-byul. Can Han-byul break free from a toxic relationship and start a new love?