Related Clues:

• Sci-fi visitor
• Foreigner
• Not from Earth
• Not terrestrial, perhaps
• Sci-fi film extra
• An American in Paris, e.g.
• Not terrestrial
• Beefeater
• Not natural
• Xenophobe's fear
• With 27-Down, sci-fi phenomena that help explain this puzzle's theme
• Border crosser
• 1979 sci-fi thriller
• Gilbert & Sullivan title character
• Nonnative