Mysterious Passenger is a game aid for gamemasters running a space opera style game where the PCs own their own ship, and regularly transport passengers. It provides 6 possible adventure seeds for a "mysterious passenger" who comes on board, but doesn't seem quite right. Some of these seeds can be the entire focus of a session, while others may merely provide a diversion on the trip to make things interesting.
Mysterious Passenger can be used with any system. For more adventure seeds involving passengers, be sure to check out 15 Space Opera Passengers.