43 ans après J.C., l'invasion romaine de ce qui va devenir le Grande-Bretagne. L'armée impériale romaine retourne écraser la Britannia, un territoire mystérieux tenu par de sauvages guerriers et de puissants druides.

Une ado orpheline se réveille à la morgue et découvre ses nouveaux super-pouvoirs : elle est porteuse du Halo d'une société secrète de religieuses chasseuses de démons.

Mocha and her two companions are adventurers for hire. Unfortunately, their quest for wealth is hampered by their inability to finish a job. Just as they are going broke, Mocha contracts to steal a mystic dragon's head from the secluded temple of fire. When the trio finally escapes with the head, it speaks, offering a fortune for abandoning the contract and taking it to a different temple. Mocha readily agrees. They soon find themselves fighting their employer to prevent the end of the world.