Intruder (1975)
A group of people arrive at an abandoned mansion on a remote island. All of them are relatives of a money mogul who is believed to have perished in a plane crash somewhere in Panama. The host, Peterson, is nowhere to be found and all the guests have individual letters upon arrival and are assigned to a specific room. The only other person there is a male maid who serves them dinner and seems to be in love with one of the guests.