Young Goo is a 7th-grade student at a special school, large and clumsy, but pure and innocent. Young Goo befriends the clever Jjang Gu, but their parents dislike their closeness and have a bad relationship. Happy summer vacation arrives, and the two families travel to an island, but realize they are unlucky to meet there and decide to take a boat to go elsewhere, but Young Goo and Jjang Gu secretly escape and remain on the island. Meanwhile, the island is used as a meeting place for international smugglers. Seung Dae and Sung Woo are part of their gang but are clumsy. They accidentally forget a national treasure ceramic on the island, are reprimanded by the boss, and return to the island to retrieve it, but the ceramic has disappeared. They think Young Goo and Jjang Gu, playing alone in a villa, have hidden it and attack the villa indiscriminately. However, Young Goo and Jjang Gu fend off these attacks with various ideas and eventually catch the smugglers.