Born from The Black Mass of Darkness, Aku is an evil, shape-shifting demon who took over the world and ruled it for countless eons. He is the arch-nemesis of Samurai Jack and the biological father of Ashi and her septuplet sisters. When Samurai Jack came to fight him, Aku tore open a portal in time and flung him into the future, where he is the de facto ruler of the world, and his evil is law. Now, the samurai called Jack seeks to undo Aku's evil and return to the past.