Error, error, error Human error Living with a chip implanted in their head, the underground city of Black Forest, where one cannot survive without dreams, is filled with the desires and errors of those who wish to fulfill their dreams. Investigator Donauverle of Petit-4 also dreams of replacing his left leg with a prosthetic while diligently trying to solve the case assigned to him, but it is not easy. Each time, there is someone who helps him: his enigmatic yet punctual roommate Haluhalo, who always pays the rent on time. Together, they correct the numerous errors in the world, one by one. Then one day, Haluhalo suddenly stops functioning after sending a message saying, "I'm sorry." Donauverle tries to revive his unconscious roommate by unraveling Haluhalo's mystery, but the deeper he digs, the more he confronts the massive and solid errors surrounding both Black Forest and Red Velvet. The one person all the clues point to is the automaton Divinity, who 'perfectly' manages Red Velvet from high above. Donauverle and his companions from Petit-4 break the barriers and taboos and head towards the place where Divinity resides, the earthly paradise of Red Velvet, which seemed forever out of reach.