A mysterious occurrence, known only as 'The Event' has caused the residents of Paradigm City to lose their memories 40 years ago. Since then, they've struggled to survive in the half-ruined metropolis. Dashing Roger Smith is officially a negotiator who handles difficult situations, but he's really a covert superhero. He's fabulously wealthy and his car and wristwatch are loaded with deadly gadgets. But when the going gets tough, Smith summons the Big O, his giant 'Megadeus' mecha to slug it out with other robots. His butler, Norman, and Dorothy, an automaton girl, assist him in these endeavours.