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 Seigi Choujins and then attack them using the Robin Memo, which contains info on all of the Seigi Choujins' weaknesses. Watching on TV all over the world, Kinnikuman and the Seigi Choujins get together and head for Planet Hydra.

Source: Kinnikuman Wiki

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Parent story: Kinnikuman