Dr. Slump found the secret treasure named "Eye of Rainbow" of legendary Nanaba Castle in the mountain. This treasure makes every dream come true. When the robbery gang of Arale, Ga-chan tried to steal this treasure, Bisna who is the descendant of Nanaba Kingdom took the treasure. Bisna wanted to revive the devil and Nanaba Kingdom. Arale and her friends went towards the secret Nanaba Castle to get back the treasure.

Source: Official Website

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Parent story: Dr. Slump: Arale-chan