On his first day working as a new teacher at his alma mater, Sunny Day Boys High School, Lee Taesung is welcomed by the bright and sunny smile of Seo Sunny, a warmhearted chemistry teacher. Feeling immediately drawn to Sunny, who resembles his first love from high school, Taesung tries to hold back his feelings by keeping his distance. But still, Taesung can't help but be pulled in by Sunny's irresistible charm...