Narumi Katou is a middle-aged man who suffers from the bizarre ZONAPHA Syndrome: a rare and inexplicable disease that causes its victims to endure severe seizures at random, with the only cure being to watch someone laugh. One day, during Narumi's part time job, a young boy with a giant suitcase fleeing from three adults runs into him. The boy introduces himself as Masaru Saiga, the new owner of the famous Saiga Enterprises following his father's recent death. However, other members of his family are trying to assassinate him and claim the fortune for themselves.