After a slumber of 12 millennia, the Demon Empire returns to seize control of the Earth. Reideen, the giant robot-like protector of the lost continent of Mu, senses the evil presence and awakens within its golden pyramid, revealing to young Japanese boy Akira Hibiki that he is the one descendant of the ancient Mu people who must help Reideen save Earth.

He was assisted by his friends, token girl Mari Sakurano (daughter of a scientist fighting the Demon Empire) and several members of his high school soccer team.

[Source: AniDB]

Hishou, Ginga, Ace, Hayate, and Ikazuchi all work at a studio doing various idol-related jobs at the request of their manager, Reiko. But in reality, these five teenagers are really the Reideens, a group of superheroes who fight the Super Devils sent by Lushu, a mysterious being. Little do the Reideens know, these Super Devils are being sent for a more important purpose...

[Source: Anime News Network]

Saiga is a normal high school student with a gift in mathematics. His daily routine is disrupted when his family gets news that his Father's remains have been discovered—a noted archeologist and researcher who had gone missing while exploring a site many years before. Among his remains were notes and artifacts that needed to be identified by the family near a notable triangular mountain in Japan known as "Japan's pyramid", a place suspected by some to be man-made. A meteor containing a strange robotic lifeform falls from the sky and begins to cause destruction, putting Saiga in danger and causing a mysterious bracelet from his father's research to activate and merge him with an ancient robot burried within the pyramid—a robot the runes describe as Reideen. It is now up to Saiga and guardian Reideen to fight against this unknown alien threat from the sky.

[Source: Anime News Network]