The president of an IT company is found dead. The cause of death is believed to be an overdose of medication for a chronic illness, but the possibility of murder emerges since potassium and other substances were found in the body. Ogawa, unfamiliar with the investigation, asks for help from Okawa, a former detective who now works for the police department. The "large-and-small duo", who are uneven in appearance and personality, take on the challenge of getting to the bottom of a difficult case.