Narumi Katou is a middle-aged man afflicted by the bizarre ZONAPHA Syndrome: a rare and inexplicable condition that causes its victims to experience severe seizures at random, with the only remedy being to watch someone laugh. One day, while working part-time, Narumi encounters a young boy with a giant suitcase who is fleeing from three adults. The boy introduces himself as Masaru Saiga, the new owner of the renowned Saiga Enterprises following his father's recent death. However, other family members are plotting to assassinate him and seize the fortune for themselves.
