Two twenty year olds, Kim Hye Rim and Lee Ki Kwang, who attended the same middle school, meet again coincidentally after many years as a college student and an idol group member. Lee Ki Kwang is a member of popular idol group falls in love with Kim Hye Rim, his old friend from same middle school who has never dated before, as the two try to keep their love a secret from the rest of the world.

「二十歲」描述成為大明星的起光，與從沒談過戀愛的女大生慧琳(李多寅 飾) 之間的浪漫故事，