Fusion is a parody on the magical girl genre. The Fusion Fighters are a group of individuals who transform with the help of Spirits, magical creatures that have a specific theme. The host and Spirit each have their own theme that if the themes work together, the stronger the bond is. But if a person and Spirit bonds through trauma or force, it will corrupt both into a monstrous beast and only take the Spirit's theme. The story follows Joey and her balance of being a Fusion Fighter and the kid of one of the leaders of the city.