The episode's story is loosely based on the Japanese folk tale of Urushima Taro. In short: Urushima Taro, a fisherman, saves a turtle from being killed. The turtle takes him off to an undersea palace where he is feasted and entertained until he has had his fill. When he leaves, he is given an ornate box and a warning to never open it. He returns home to discover that decades have passed, yet he hasn't aged. Finally, curiosity is too much for Urushima Taro, who opens the box, to find that it contains his old age.
Also known as "Lupin in Paradise."
Adapted from Chapter 4 of 'Lupin III: World's Most Wanted', titled 'Love Thy Sister'.