I recently watched this one again since it was on TCM and it was part of Quint's Movie A Day series on AICN. Director Alfred Hitchcock often said this was his favorite film, a sentiment shared by the movie's female lead Teresa Wright. It's a great movie, but not my favorite Hitch.
The story revolves around the Newton family. Charlie Oackley (Joseph Cotten) comes to visit the family and his niece Charlie (Wright). Through the course of the movie, Young Charlie begins to suspect her Uncle Charlie is a serial killer known as "The Merry Widow Murderer". The movie is chocked full of twists and turns and a great deal of suspense by the undisputed master of cinematic suspense.
Keep a close eye out for a young Hume Cronyn as Young Charlie's father's best friend.