The story takes place between the first and second seasons, where Rimuru confronts the swirling conspiracies in the Coleus kingdom.

In the year 2138, virtual reality gaming is booming. Yggdrasil, a popular online game is quietly shut down one day. However, one player named Momonga decides to not log out. Momonga is then transformed into the image of a skeleton as "the most powerful wizard." The world continues to change, with non-player characters (NPCs) beginning to feel emotion. Having no parents, friends, or place in society, this ordinary young man Momonga then strives to take over the new world the game has become.

Tomoya Okazaki is a third year high school student resentful of his life. His mother passed away from a car accident when he was younger, causing his father to resort to alcohol and cigarettes. This results in fights between the two until Tomoya's shoulder is injured in a fight. Since then, Tomoya has had distant relationships with his father, causing him to become a delinquent over time. While on a walk to school, he meets a strange girl named Nagisa Furukawa who is a year older, but is repeating due to illness. Due to this, she is often alone as most of her friends have moved on. The two begin hanging out and slowly, as time goes by, Tomoya finds his life shifting in a new direction. Anime based on a popular visual novel game.