partially taken from Daily Yomiuri online
A famous Japanese fairy tale similar to Rip Van Winkle. The story starts with a "usual once upon a time"...
Long, long ago, a young fisherman named Urashima Taro saw some children poking a turtle at the seashore. Moved by the turtle's plight, Taro persuaded the children to let it go.
Later, when he was fishing, a turtle appeared and said: "I'm the one you rescued. In gratitude, I'd like to take you to the Dragon Palace." The turtle put Taro on its back and swam to a glittering palace at the sea bottom. There, the daughter of the Dragon King, Oto-hime, thanked Taro for saving her retainer.
After three years, Taro wanted to return to his village. With sadness, the princess gave him a lacquered box and said: "If you want to return someday, never open this box."
When Taro returned home, he found a desert where his house once stood. He asked an old woman passing by what had happened. "The Urashima family? I heard they once lived here, but their only son went fishing one day and never returned," she said. "They say that was 300 years ago."
Dazed, Taro forgot the princess' warning and lifted the lid of the box. White smoke rose, and he turned into an old man.
Relevance to Urusei Yatsura
This fairytale is played upon constantly in Urusei Yatsura. Most notably the second movie "Beautiful Dreamer" which has scenes including Tomobiki town being upon a turtle's back among other symbolic references. There are also several TV episodes that spoof Urashima Taro. There's almost too many to name all the references. Off the top of my head, there's "The Great Year-end Class Party" in which Onsen-Mark takes up the role of Urashima Taro.