Na-mi is the new girl at school who just moved from the countryside. Whenever she's nervous she can't control her accent and one day one she is laughed at by the school bullies.Just then, a group of girls jump in to help. They are Chun-hwa, the most righteous girl in Jindeok Girl's High, unattractive Jang-mi who is obsessed with big eyes, Jin-hee the cursing queen, literature-loving girl Geum-ok and her monstrous powers, peculiar Bok-hee, who dreams of winning the beauty pageant, and cold, arrogant Suji. Na-mi joins them to form group 'SUNNY', promising each other they will stay together forever. But a sudden accident tears them apart. 25 years later, married and living with a fine husband and a beautiful daughter, Na-mi still feels something lacks from her life. One day, she runs into Chun-hwa and feeling joy from the encounter she decides to find other members of SUNNY too. Stepping out of everyday life devoted to her family, Na-mi begins her search of friends from the past, and remembering the dazzling friendship of the time she finds herself at the happiest moment of her life.