The plot involves Rimuru's journey from being a rather weak (initially) slime to becoming a very powerful being. He meets many interesting characters. For example, there are the goblins he befriends early on. Rimuru helps them improve their living conditions. As the story progresses, he also has to deal with the threat of other more powerful monsters and even some human kingdoms that view his growing power with suspicion. In general, it's a story full of adventure, growth, and the forging of relationships in a magical and dangerous world.
A great aspect of an Arturia and Muramasa fanfic is the potential for world - building. Since they come from different settings, the fanfic can create a new world that combines elements of both of their original worlds. This could include unique magic systems, different cultural elements, and new types of creatures. The relationship between Arturia and Muramasa can be the centerpiece that drives the exploration of this new world, with their growing bond or conflict shaping the very fabric of the fictional universe.
The main character is Rimuru Tempest. He is a slime who has unique powers and a great deal of charisma. Another important character is Veldora, the storm dragon who befriends Rimuru. There is also Shizu, a human who has an impact on Rimuru's early development. And then there are characters like Benimaru, a powerful ogre who becomes one of Rimuru's loyal followers.
The Tensura web novel follows the story of Rimuru Tempest. He starts as a normal salaryman in Japan but is reincarnated in a fantasy world as a slime. Rimuru then begins to build his own nation and gathers various powerful and unique beings around him. He also gets involved in many power struggles and battles against other strong entities in this new world.