Kratos is a warrior who serves the Greek gods of Olympus. Flashbacks reveal that he was once a successful captain in the Spartan army and led his men to several victories before being defeated by a barbarian king. Facing death, Kratos called on the God of War, Ares, whom he promised to serve if the god would spare his men and provide the power to destroy their enemies. Ares agreed and bonded the Blades of Chaos, a pair of chained blades forged in the depths of Tartarus, to his new servant. Kratos, equipped with the blades, then decapitated the barbarian king.

Kratos waged war at the behest of Ares, eventually leading an attack on a village occupied by worshipers of Athena. Unknown to Kratos, Ares had secretly transported Kratos' wife and daughter to the village; during his frenzied attack on its temple, Kratos accidentally killed them. Although Ares believed this act would free Kratos to become the perfect warrior, the Spartan instead renounced his pledge of servitude to the god. The oracle of the destroyed village cursed Kratos by bonding the ashes of his dead family to his skin, turning it ash-white and earning him the nickname, "Ghost of Sparta". Plagued by nightmares of his horrible deed, Kratos vowed to serve the other gods in hope of ridding himself of the visions.

When the game starts, Kratos has been serving the gods for ten years. He kills the Hydra on behalf of Poseidon, but he has grown tired of his service and suffering. He summons Athena, who states that if Kratos performs one final act—the murder of Ares—he will be forgiven for killing his family. Ares is waging war on the city of Athens out of hatred and jealousy of his sister Athena, who assigns Kratos to destroy Ares because Zeus has forbidden divine intervention. Athena guides Kratos to the war-torn Athens. After a strange encounter with a gravedigger who encourages him to continue his task, Kratos battles his way to Athens's oracle, finds her, and learns that the only way to defeat Ares is with Pandora's Box, a mythical artifact that grants the power to kill a god.