Calla Westcott was originally a little angel from Heaven. After being reincarnated as a human girl, she lost her former memories and retained only her innate kindness and an unusually keen perception of others' pain. But her childhood in the human world was not happy. Her mother died early, and her biological father, Gavin Reeves, regarded her as a burden and never truly gave her a home.
One day, in order to save a stray puppy that suddenly ran onto the road, Calla disregarded danger and lunged into the traffic lane, where she was struck by a失控 car and her heart briefly stopped. When she opened her eyes again, she had arrived at the Heavenly Garden and met Mother Mary. Only then did Calla learn that she had once been a little angel. Mother Mary prepared to take her back to Heaven, but for the first time, Calla refused. She was unwilling to end her short life just like that, because she had never truly had a home, nor did she know what it felt like for someone to be unwilling to let her go.
Mother Mary gave her one last chance: Calla could return to the human world for thirty days. If, before the end of the thirtieth day, she could receive kisses given to her sincerely from the hearts of five different people, she could continue to stay in the human world. The kisses could only land on her forehead or cheek, and she could not actively ask for them, exchange for them, force them, or deliberately perform for the sake of completing the contract.
