"The Adventure of the Blue Carbuncle" is a short story by Arthur Conan Doyle featuring the famous detective Sherlock Holmes. The plot begins when Holmes receives a lost hat and a goose from a man who found them after a scuffle. Inside the goose, they discover a valuable gemstone, the blue carbuncle, which has been stolen. Holmes investigates the origins of the hat and the goose, leading him to uncover the story of a man named James Ryder, who had stolen the gem. Ultimately, Holmes decides to let Ryder go, believing that justice can sometimes be served with mercy.