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Kaito, Taki, and Rena are swept away by a whirlpool and end up in an underwater city full of life force. The city inhabitants attack them and confiscate Kaito's Dino Dagger. A girl named Helen rescues them and tells them they are in Atlantis. Diamond Ryugu sends the Drago Clones to the city to get the life force. Kaito and Rena must stop the Drago Clones by reviving Atlantis' guardian, Dino Icthyo.