In ancient times, the Titan Cronos (son of Uranus the sky and Gaia the earth) and his wife Rhea gave birth to six children. Five of them were swallowed by Cronos, and only one was saved, Zeus, hidden from Cronos by Rhea. Upon reaching adulthood, Zeus forces his father to vomit the other five gods. Zeus, along with his brothers and sisters, decide to battle the Titans for control of the universe. Cronos and the other eleven Titans lose the battle and are sent to Tartarus.

Thousands of year later, in Greece, the Pope visits the room of the recently reincarnated infant Athena, with a golden dagger. Before he can kill her, the Gold Saint Saggitarius Aiolos saves her, and unmasks the Pope. Aiolos, horrified, flees. The Pope declares him a traitor to the Sanctuary, and has Capricorn Shura murder him. This causes his younger brother, Leo Aiolia, to grow under the shadow of his "traitor" brother, suffering from all manner of abuse and scorn. This leaves him with a grudge against the Gold Saints and Sanctuary as a whole, which causes the Pope and most of the Sanctuary to distrust him. To make things worse, the enemies of God Zeus, the Titans, are revived and begin to seek vengeance against him and the Sanctuary. This marks the start of Aiola's story and the battle of the Gold Saints against the Titans.

(Source: Wikipedia)