Mouse follows Jeong Ba-reum (Lee Seung-gi), a steadfast police officer, whose life changes when he encounters a psychopathic serial killer. This drives him and his partner Go Moo-chi (Lee Hee-joon) to unearth the truth behind psychopathic behaviours. This also raises the questions, whether it is possible to scan a psychopath in the mother's womb using fetal genetic testing? And if the child in the womb is a psychopath, will it be wise to have that child?