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 newly formed Gravion faces off in their first battle against the Zeravire and the pilots discover their weaknesses. Meanwhile The Earth Federation Alliance plan their own attacks on the Zeravire.

Eiji begins to get used to life in Sandman's castle and to the castles inhabitants. As he and Toga venture to the forbidden area of the castle in search of Eiji's sister Ayaka and the identity of the G-Shadow's pilot, Raven attempts to convince the EFA to allow Earthgertz to take the lead in the defense against the Zeravire.

When a ghostly figure is sighted in one of the castle's towers, Toga and Eiji go to investigate. There they discover Leele, the mysterious pilot of the G-Shadow.

In an effort to get to know each other better, the team goes on a picnic together but Leele is hesitant to share as she is haunted by nightmares of her past. Meanwhile the EFA attempts to find information of Sandman.

Eiji sneaks out of the castle during the staffs shopping day and takes Toga along with him for his first day out of the castle and his first day in the big city. As the maids search for them, the Zeravire launch an attack on the city.

On the anniversary of her father Isanas death, Luna returns home to the island of Okiaowa but is surpirsed when she finds that the rest of of the Gran Knights have decided to accompany her to the have some fun on the island. But all fun is soon ended when the Zeravire target the International Peace Conference for their next attack.

When the Zeravire attacks the the castle, all hope of stopping it lies with Eiji, who has accidentally found himself piloting the Gran Kaiser. To survive he must call upon all the power of the Gran Kaiser and all that his body can withstand.

Repairs begin on the castle following the attack by the Zeravire and Eiji begins to recover from his battle but runs from his physical and runs into Mizuki who gives him insight to the disappearance of his sister Ayaka. Together they decide to investigate where she vanished, the West Tower.

After seeing what darkness Toga is capable of in the heat of battle, Eiji quits the Gran Knights and leaves the castle to return to a normal life. Leele ventures to town to convince Eiji to come back. Two pilots down, Gravion must operate with its auto-pilot taking up the slack when the Zeravire attack.

The battle wages on as Gravion fights against the attacking Zeravire with a failing auto-pilot and Eiji and Leele still missing. But after his friend Yumi is injured in the attack, Eiji decides that his place is with the rest of the Gran Knights on Gravion.

The Zeravire have been evolving, not in order to destroy humans, but to ensure the destruction of Gravion. When the new and improved Zeravire comes to Earth to challenge Gravion, Sandman is hesitant to let the Gran Knights go to battle but they refuse to sit on the sidelines. With no other choice, Sandman must authorize the use of Gravions most adavanced weapon.