Action Mask's 'Action Stone' has been robbed by Leotard Devil. Without the Action Stone, he can't go to his world. So he chooses Shin-chan and his family to get back his stone through the Action Card numbered 99 (the most precious card in the whole set of choco chips.) According to the plan of Action Mask and Ririko, they make Shin-chan's family to move to a beach so they can make them travel to the parallel world so they could help them to defeat the Leotard Devil.

Source: Wiki

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Parent story: Crayon Shin-chan