While searching for his missing sister, Eiji Shigure secretly enters the castle of millionaire Klein Sandman. There he meets the castles strange inhabitants and is selected to join in Sandmans mission to stop a destructive alien known as the Zeravire. The teams weapon, a series of fighting robots designed by Sandman called the Gran Kaiser and the Gran Divas who when needed join together to form Gravion.

The Gravion team completes another mission, mucht to the delight of the president. Someone says the Zerevie are going to return at any moment. The team is made to be maids, due to a strip mah jongg game that got out of control. They are forced to cook, clean, do the laundry, repair robots, and more. Eiji is forced to wear a dress.

Meanwhile, Mr. Sandman is meeting with someone elsewhere. Back at the mansion, Luna shows off Toga's room when he was a child to Eiji. Then the G-alert sounds. The Grand Knights go to intercept the intruder. After much destruction, they decide to combine, but cannot. He's stuck in a subway. After trashing a street punk, Sandman shows up and lets them combine into Gravion. A lot of chaos and distruction ensues, ending with the Zerieve being destroyed.

Elsewhere, some generic sinister people are plotting to reshape the earth...