Cheerful and mischievous, Moon Haet-nim is a freelance voice actor. She voices the main character, Happy Mong, in the lively children's animation Detective Happy Mong—a role that perfectly fits her personality. One day, while accompanying her friend Young-hee, who is struggling with depression, to a psychiatric clinic, Haet-nim takes a test out of boredom... The result? Depression?! This can't be real! In denial, Haet-nim embarks on a journey, just like her animated character, to uncover the cause of her depression!