As the whole world watches the Choujin Rugby game on TV, the "Underground Space Emperor" Hydra King has his Underground Space Supermen kidnap Robin Mask. Their aim is to lure out the Justice Choujins and then attack them using the Robin Memo, which contains info on all of the Justice Choujin's weaknesses. Watching on TV all over the world, Kinnikuman and the Justice Choujins get together and head for Planet Hydra.