In the second book of Plato's philosophical work The Republic, Socrates encounters a man named Glaucon who uses a mythological story to prove a point about human nature. Ultimately, Glaucon and Socrates have very different interpretions of the same tale.
The story of Gyges's ring was a well-known myth before Plato used it in his book. It told of a man named Gyges who lived in Lydia, an area in modern Turkey. He was a shepherd for the king of that land. One day, there was an earthquake while Gyges was out in the fields, and he noticed that a new cave had opened up in a rock face. When he went in to see what was there, he noticed a gold ring on the finger of a former king who had been buried in the cave. He took the ring away with him and soon discovered that it allowed the wearer to become invisible. The next time he went to the palace to give the king a report about his sheep, he put the ring on, seduced the queen, killed the king, and took control of the palace.