The world of 'Nightmare Odyssey' resembles our own, where a mysterious criminal organization known as 'The Dark Order' has seized control of society. They have established a reign of terror, stifling freedom and causing chaos. However, a determined faction of rebels, the 'Voyagers of Freedom,' has risen to fight against this tyranny. The Dark Order has created virtual arenas where the Voyagers of Freedom are forced to combat waves of enemies for their own entertainment. At the end of each wave, the Voyagers of Freedom have the opportunity to choose from three skill upgrades in the form of cards. However, if any of them is defeated in the arena, they must restart their journey from the beginning.