Axel Callahan works as a professional assassin. He's been paid by many people to assassinate their enemies or bad people. However, he never enjoyed his job. This is not due to a lack of payments or whatever. He doesn't feel the spark and enjoyment while executing his job. Not until he met his girlfriend. She succeeds in filling the empty void in his heart and mind by letting him join her as her serial killer partner. From that moment on, Axel felt his soul become more alive and energetic.