Two siblings find a tomb in their house and unleash a sinister creature using a jewel with cosmic powers, turning it into their pet monster. The siblings play a game that leads to the monster's emergence. The creature defeats thieves in an abandoned warehouse and is controlled by the shiny gem. The siblings discover human remains and the monster attacks them, but one of the siblings, Mimi, commands it to stop. The monster reveals it is the Arch Duke of Nightmares, and Mimi names him Psycho Gorman. They leave the monster in the factory and plan to return.