The Five Orange Pips

A young gentleman named John Openshaw visits Holmes one night with a strange story. His uncle Elias had emigrated to the United States as a young man, establishing himself as a planter in Florida and joining the Confederate States Army, in which he rose to the rank of colonel. In 1869 or 1870, he returned to England and purchased an estate near Horsham in Sussex, adopting a reclusive lifestyle. He later allowed John to live on the estate and gave him full access to the property, except for one storage room that was always kept locked. On 10 March 1883, Elias received a letter postmarked Pondicherry, bearing the inscription "K.K.K." and containing only five orange pips (seeds). What happens after that you will need to listen to find out.