"Tensei Shitara Slime Datta Ken: Koriusu no Yume" is a side-story anime adaptation set within the universe of "That Time I Got Reincarnated as a Slime." The narrative centers on Rimuru Tempest, a slime who was reincarnated into a fantasy world after dying in his previous life. Rimuru, now the powerful and benevolent ruler of the Jura Tempest Federation, encounters a mysterious girl named Koriusu, who appears in Tempest under unusual circumstances. Koriusu possesses a unique ability to manipulate dreams, which becomes the focal point of the story.

The plot unfolds as Rimuru and his allies investigate Koriusu's origins and the nature of her powers. Koriusu's abilities inadvertently cause disturbances in Tempest, creating dream-like scenarios that blur the line between reality and illusion. These disturbances attract the attention of both allies and adversaries, leading to a series of events that test Rimuru's leadership and problem-solving skills. The story explores themes of trust, cooperation, and the consequences of unchecked power, as Rimuru works to stabilize the situation and uncover the truth behind Koriusu's arrival.

Key characters include Rimuru Tempest, whose calm and strategic demeanor drives much of the narrative; Koriusu, whose mysterious presence and dream-manipulating abilities serve as the catalyst for the story's events; and Rimuru's trusted companions, such as Shion, Shuna, and Benimaru, who play supporting roles in addressing the challenges posed by Koriusu's powers. The interactions between these characters highlight the dynamics of the Jura Tempest Federation and its commitment to harmony and mutual understanding.

The side-story maintains the lighthearted and adventurous tone of the main series while introducing new elements that expand the world-building. It provides additional insight into Rimuru's character and his approach to leadership, as well as the diverse abilities and personalities of those around him. The narrative is self-contained, offering a fresh perspective on the established universe without requiring extensive knowledge of the main series.