From Wikipedia: Urashima Taro

I got lazy and found the Wiki entry for the story instead of finding the Japanese language one floating around in my library somewhere.

The following is a direct cut and paste.  Love the pics. ^^

One day a young fisherman named Urashima Tarō is fishing when he notices a group of children torturing a small turtle. Tarō saves it and lets it to go back to the sea. The next day, a huge turtle approaches him and tells him that the small turtle he had saved is the daughter of the Emperor of the Sea, Ryūjin, who wants to see him to thank him. The turtle magically gives Tarō gills and brings him to the bottom of the sea, to the Palace of the Dragon God (Ryūgū-jō). There he meets the Emperor and the small turtle, who was now a lovely princess, Otohime.

Urashima Tarō illustration by Edmund Dulac

Tarō stays there with her for a few days, but soon wants to go back to his village and see his aging mother, so he asks Otohime permission to leave. The princess says she is sorry to see him go, but wishes him well and gives him a mysterious box called tamatebako which will protect him from harm but which she tells him never to open. Tarō grabs the box, jumps on the back of the same turtle that had brought him there, and soon is at the seashore.

When he goes home, everything has changed. His home is gone, his mother has vanished, and the people he knew are nowhere to be seen. He asks if anybody knows a man called Urashima Tarō. They answer that they had heard someone of that name had vanished at sea long ago. He discovers that 300 years have passed since the day he left for the bottom of the sea. Struck by grief, he absent-mindedly opens the box the princess had given him, from which bursts forth a cloud of white smoke. He is suddenly aged, his beard long and white, and his back bent. From the sea comes the sad, sweet voice of the princess: "I told you not to open that box. In it was your old age ..."

Variations

As always with folklore, there are many different versions of this extremely famous story. In one, for example, there were three drawers in the box. After he turned into an old man he found a mirror, then took the body of a crane when touched by a crane feather from the last box, in another he ate a magic pill that gave him the ability to breathe underwater. In another version, he is swept away by a storm before he can rescue the turtle. Also, there is a version in which he dies in the process of aging (his body turns into dust), as no one can live 300 years.

Commemoration

Statue of Urashima Tarō in Mitoyo, Kagawa

A shrine on the western coast of the Tango Peninsula in northern Kyoto Prefecture, named Urashima Jinja, contains an old document describing a man, Urashimako, who left his land in 478 A.D. and visited a land where people never die. He returned in 825 A.D. with a Tamatebako. Ten days later he opened the box, and a cloud of white smoke was released, turning Urashimako into an old man.

Later that year, after hearing the story, Emperor Junna ordered Ono no Takamura to build a shrine to commemorate Urashimako's strange voyage, and to house the Tamatebako and the spirit of Urashimako.