Kuroha Yoitsuji has always been told that she possesses extraordinarily good luck, but as a result, it seems that bad luck spreads to those around her. Orphaned at a young age, she is cared for by the leader of the Yakuza group "Hibauchi," who uses her as a living weapon against his enemies. One day, in despair at the misfortune she causes others, Kuroha throws herself out a window to escape her guardian and captor. However, before she can hit the ground, she is rescued by a youth named Nijisuke Murokawa.

Although Nijisuke has been the victim of an unbelievable number of accidents in his life, he refuses to believe in luck—whether it be good or bad. Insisting that all events are the result of mere coincidences, his mental and physical strengths serve as an inspiration to Kuroha. As she attempts to leave the city with the help of Nijisuke and his mysterious companion Hokuto, she begins to wonder if it is her savior who is really strong or if it is her who is very weak. Is it possible for her to take control of her own fate and stop being a tool for others?