Dr. Slump comes across the Eye of Rainbow, a secret treasure belonging to the legendary Nanaba Castle up in the mountains. The treasure is said to make every dream come true. When a band of robbers goes after it along with Arale and Ga-chan, Bisna, a descendant of the Nanaba Kingdom, gets hold of the treasure instead. Bisna wants to bring back a devil and restore the Nanaba Kingdom. Arale and her friends set out for the hidden Nanaba Castle to recover the treasure. The film is an adventure comedy.