Castle in the Desert (1942)

Paul Manderley, eccentric historian, and his wife, descendant of the Borgias, live in an isolated castle-like mansion in the Mojave Desert. When a guest suddenly collapses, Charlie Chan is invited to stay. As the standard mystery-mansion props come into play, and all means of outside communication are sabotaged, it becomes evident that one of the inhabitants has access to poisons and is prepared to use them...