Urashima San stays in the the Palace of the Dragon King of the Sea(the Rin Gin) for three days, forgetting about his parents.

300 years pass above the sea in Urashima's village and his parents die.

Urashima doesn't know this and leaves the palace and his wife to go back to his village, bringing the box the princess gave to him.

After finding out what happened, in his despair and sorrow, he breaks his promise with the princess and opens the box.

This causes him to age 300 years and die from old age. He can never go back to his beloved wife or the palace ever again.