贤淑（金喜爱 饰）靠着在超市工作所得的微薄收入艰难抚养着两个女儿万智（高智星 饰）和天智（金香奇 饰），两个女孩虽然身为姐妹，却拥有着截然相反的性格。万智个性孤僻而叛逆，和家人之间关系十分冷淡；天智则乖巧可爱，常常想着法子逗妈妈开心。 　　让贤淑和万智没有想到的是，乐观开朗的天智竟然在某一日选择了结束自己的生命，这一噩耗犹如一道晴空霹雳打响在贤淑一家人的头顶之上，使得本就辛苦的生活变得更加举步维艰。然而，即便如此，贤淑和万智还是坚强的维系着正常的生活。一次偶然中，万智意外的得知了有关妹妹生前的一些讯息，这些讯息让她明白了妹妹真正的死因和竭力隐藏的某个秘密。Because of middle schooler Cheon-ji's unexpected suicide, her mother Hyun-sook and older sister Man-ji are driven out of their home and move to another apartment. By chance, Man-ji hears a story that might relate to the suicide after meeting her dead sister's friends. Tracking down clues, Man-ji discovers that Cheon-ji's best friend, Hwa-yeon is behind all the events that happened. While seeking for the secret behind her sister's death, Man-ji finds out that Cheon-ji left notes in red knitting balls.