"Reideen the Brave" centers around Junki Saiga, a high school student with a passion for music, who becomes entangled in a series of events involving ancient technology and extraterrestrial threats. While visiting his father's archaeological site, Junki encounters a mysterious artifact known as Reideen, a colossal robotic entity with ties to an ancient civilization. This discovery coincides with the arrival of alien beings called the Zorima, who begin attacking Earth. Junki is chosen as the pilot of Reideen, a role that connects him to the machine through a psychic link, granting him the ability to control its immense power.

As the story progresses, Junki learns that Reideen is not merely a weapon but a sentient being with its own will and purpose. The Zorima, led by their enigmatic commander, seek to harness Reideen's energy for their own destructive goals. Junki must navigate the challenges of piloting Reideen while balancing his personal life, including his relationships with friends and family. His journey is marked by battles against increasingly powerful Zorima forces, as well as internal struggles to understand Reideen's origins and his own role in the conflict.

Supporting characters include Akira Hibiki, a skilled pilot and Junki's childhood friend, who provides both emotional support and tactical assistance. Professor Saiga, Junki's father, plays a crucial role in uncovering the secrets of Reideen and the ancient civilization that created it. The series also introduces other pilots and allies who join Junki in the fight against the Zorima, each contributing unique skills and perspectives to the ongoing battle.

The narrative explores themes of responsibility, sacrifice, and the coexistence of humanity and technology. As Junki grows into his role as Reideen's pilot, he grapples with the moral implications of wielding such power and the toll it takes on his personal life. The series balances intense mecha battles with moments of introspection, delving into the psychological and emotional challenges faced by its characters. Through its blend of action, mystery, and character-driven storytelling, "Reideen the Brave" presents a compelling tale of courage and resilience in the face of overwhelming odds.