The cyborgs are no longer on duty, they live normal lives until a mysterious space ship lands on earth. The only survivor on the ship warns about an evil conqueror called Zoa that wants to conquer the universe, which means the cyborgs are back in action...

Away from home for 10 years, Azuma Kazuki had no idea what was awaiting him upon his return. He certainly did not expect to be attacked by dozens of heavily armed people and taken as their prisoner. Fortunately, Azuma's stay in captivity is short. Wielding a strange sentient weapon—known as "Bubuki"—upon her right arm, his childhood friend, Kogane Asabuki, rescues him. After escaping, Azuma learns that he too has the power to control Bubuki. Together with his new companions, he must revive the Buranki titan named Oubu, that sleeps somewhere deep underground. With this knowledge and purpose, a new path to unearth the truths behind the Buranki opens before Azuma. [Written by MAL Rewrite]

In ancient times, the continents of Atlantis and Mu fought a war, and Atlantis lost. After losing the war, Atlantis transformed itself into a small planet and timeslipped to the present-day to try to take over the world again. But five warriors from Mu assemble on Easter Island, and together with giant cyborg white whale that houses the brain of the ancient queen of Mu, fight Atlantis off at various famous ruins around the world.

Summary for the Gosenzosama Banbanzai! OVA.