天才科学家天树浩一向美树求婚的时候，突然从天上飞来了一架宇宙飞船，上面载着一个几岁的小孩子，他们把她收养了，改名叫天树光。而在宇宙中，有一群人正寻找“皇室盔甲”，天树光正是盔甲的继承人……原来天树光是拥有古拉星王家血统的公主，其后她穿上古拉星王位继承者之证“皇室盔甲”，变身为GranDoll而同企图夺取皇室盔甲的敌人战斗。

Hikaru has always believed herself to be just a normal high school girl...well, as normal as a teenager with a genius mad scientist for a father. She spends her time with her best friend, daydreaming about the boy she loves and watching her favorite superhero show. One day, the past that she has known nothing about comes back to haunt her. She is shocked to learn that she is the last surviving member of the royal line from a faraway planet. Even more, she finds out that she has been protecting the ultimate armor and that it can decide the fate of the entire galaxy.

ドジでおっちょこちょいでおたくだけど、ちょっとかわいい女の子・天樹ひかる。だが実は彼女は宇宙の彼方にあるグランナース星王家の血を引く姫だった! グランナース星の王位継承者の証「王家の鎧(グランスーツ)」を装着し、グランドールに変身したひかるは「王家の鎧」を奪おうとする宇宙からの刺客に立ち向かう!!