From the rulebook:
Previously, in Guardian Heroes...
The world suffered under the bitter reign of Kanon, the black mage, and his powerful Magic Kingdom. But the evil sorcerer was not satisfied and hungered for the mighty Soul Sword--the blade containing the soul of the Legendary Undead Hero, savior of the world in ages past.
He ordered his men to seize the sword, which was closely guarded by four noble adventurers in hiding--Han, Randy, Ginjirou, and Nicole. When the black mage's Royal Red Knights caught up with the adventureers, the Knights' commander Cerena disobeyed Kanon's order and came to the hidden nobles' aid. Together they revived the Undead Hero and with his help, began a war against Kanon and his malicous forces. Soon they were joined by Valgar, prince of the Magic Kingdom and head of the Royal Black Knights, who had been betrayed by Kanon and sought revenge.
When Cerena, Valgar, and the four heroes were called up to the heavens at last, they discovered that their struggles was far from over.The battle was all part of a greater plan to create ultimate warriors. For the mighty heavens, the mortal world was merely a proving ground. To make matters worse, the Undead Hero gave up control of the Soul Sword and returned to eternal rest. Without him, the final battle against the heavens became nearly impossible. Hwne the gods asked the new heroes to become their servants as ultimate warriors, some refused, some accepted, and some escaped back to the earth.
A long time passed...
Now, in Advance Guardian Heroes, the immortal Zur appears from nowhere, enlisted the heroes as servants of the gods and reviving Kanon with his special skills. Zur orders the heroes' souls to be pulled out, paving the way for Kanon to rebuild his Magic Kingdom and bring the peaceful world back under his malevolent control.
The surviving royal soldiers form a resistance against Kanon, but their numbers are rapidly decimated by the powerful mage. In a last, despirate attempt, they summon the power of the Soul Sword and ask for help from the soul of a hero from the other world. But a soul cannot enter the world without a body to accept it, and so an unknown soldiers volunteers himself as the sacrifice.
Now, together with the Princess, the unknown soldier heads to once again take control of the Legendary Soul Sword. To become a Legend... and save the earth...