Joe Blow (or Jo Blow if you prefer a female protagonist) living a successful, dynamic life in your city, slips through a time warp and comes out in another time (or place).
Past or future is up to you.
How does Joe cope? Are his 21st century skills an advantage or a disadvantage where he ends up? How does he get back? Or doesn't he?
how about a man who meets a mermaid in New York City? Only he thinks she's human.