On a chilly October evening in 1786, a mysterious stranger knocked on the door of the house of the famous English portrait painter Joshua Pope. A woman is ready to give away a magnificent jewel - an emerald necklace in the form of a skillfully made coiled snake - in exchange for the story of a portrait he painted 20 years ago. The artist refuses the necklace, which awakens terrible memories in him, but at the same time promises to tell her everything he knows about the events of which he was an involuntary witness. But for this the woman must identify herself and tell how the ominous emerald snake with a ruby head ended up in her possession.