There are currently many cases mixing existing multiple stories and their characters. A new special world is constructed by extending existing independent stories and a new mixed story in the special world provides audience vivid fresh experiences. However, if the mixed story loses the reality of the popular existing stories, it becomes unattractive to the audience. In this paper, we present a guideline when designing a good mixed story from multiple existing stories by analyzing a popular game title named Super Smash Bros., which contains a mixed story derived from other popular games' stories. We believe that our guideline is useful when we design the services integrating various media presenting respective stories through the transmedia storytelling technique.