"Mashin Eiyūden Wataru: Sōkaizan Eiyū Densetsu" is a fantasy adventure anime centered around a young boy named Wataru Ikusabe, who is transported to a parallel world called Soukaizan. This world is in turmoil, ruled by a tyrannical emperor named Daimaō, who seeks to conquer both Soukaizan and the human world. Wataru discovers that he is the chosen hero destined to save Soukaizan by retrieving the four elemental crests, which hold the power to defeat Daimaō and restore peace.

Wataru is guided by a dragon named Shibaild, who serves as his mentor and protector. Together, they embark on a journey across Soukaizan, encountering allies and adversaries alike. Among his companions are Hinata, a skilled warrior, and Ryujimaru, a young boy who wields a powerful sword. Wataru also gains the ability to summon and control mechanical dragons known as Mashin, which are crucial in battles against Daimaō's forces. Each Mashin represents one of the four elements—fire, water, earth, and wind—and Wataru must master their powers to progress in his quest.

The narrative is structured around Wataru's efforts to locate and secure the elemental crests, each guarded by formidable foes. Along the way, he faces moral dilemmas, personal growth, and the challenges of leadership. The story explores themes of courage, friendship, and the struggle between good and evil, as Wataru learns to balance his responsibilities as a hero with his own humanity.

Daimaō's forces, led by his generals, constantly attempt to thwart Wataru's mission, leading to numerous confrontations and strategic battles. As Wataru gathers the crests, he uncovers the truth about Daimaō's origins and the deeper connection between Soukaizan and the human world. The climax of the series revolves around a final showdown between Wataru and Daimaō, where the fate of both worlds hangs in the balance.

The anime blends action, fantasy, and character-driven storytelling, with a focus on Wataru's journey from an ordinary boy to a legendary hero. The series emphasizes teamwork, perseverance, and the importance of standing up against oppression, making it a compelling tale of adventure and heroism.