In a totally creative twist on the story of Persephone and Hades, we find our characters in the future. In modern day Athens, Cerberus has escaped the Underworld and ran off, leaving Hades to find him before he can wreak havoc on the mortal realm. Only, he needn't have worried, because when he does find Cerberus, everything's fine. The large, three headed beast is getting belly rubs from a demigod, who is completely unaware of her divine parentage.
That's another main twist, lmao. Persephone is a demigod, still very much the daughter of Demeter, just with a human father.