The story takes place between the first and second season. Rimuru performs a special mission given to him by Yuuki Kagurazaka. The King of the Kingdom of Coleus has reached old age and the battle for the succession of the throne among the royal family has begun. The main contenders are First Prince Sauzer who is backed by the nobility and Second Prince Aslan who is backed by the local branch of the Free Guild which has gone rogue and declared their allegiance to him. It's up to Rimuru to infiltrate the Kingdom of Coleus undercover as the Great Phantom Thief Satoru and deal with the situation, while also uncovering a secret plot brewing in the shadows.