Sherlock Holmes is approached by Mr. J. Neil Gibson, an American mining magnate. His wife has been murdered and the governess, Grace Dunbar, arrested for her murder. There is a large weight of evidence against Ms Dunbar but Mr Gibson is convinced of her innocence and wants Holmes to ensure that she is exonerated.
A very intriguing episode. The evidence against Ms Dunbar is too substantial to ignore but she does not appear to be the murderer. What is the connection between Mr Gibson and Ms Dunbar? If Ms Dunbar didn't do it then surely Mr Gibson did? But why would he hire Holmes then as if he is guilty surely Holmes will figure that out, and Gibson knows this?
The solution is one of the more original ones in the Sherlock Holmes canon.