An easy trope to explain: A perfectly normal Everytown from Anywhere, America, is mysteriously transported somewhere and/or somewhen
else. It could be an alien world, the far future, the past, a parallel universe with a completely different history or a fantasy dimension where magic works and dragons patrol the sky.
Then the Plucky Americans
(which if it isn't a trope, should be) must pull together, cooperate and arm themselves to survive. I repeat, arm themselves.
On which score it's a good thing that it's an American town and not, say, a Canadian or Danish town, because the local firepower quotient would be much lower.
Town Out of Time is primarily a literary trope. I don't think I've ever seen it done on TV or in film, though I'm sure it must have been at some point.
A subtrope of Mass Teleportation
and Trapped in the Past
(which has a subsection on entire groups being transported).
- Green Lantern: Mosaic.
- The town of Marshville in the Cavewoman comics.
- In Secret Wars, the Beyonder creates Battleworld out of various pieces of worlds throughout the universe, including a suburb of Denver Colorado.
- 1632 and sequels by Eric Flint: The West Virginia coal town of Grantville ends up in 16th century Germany. It has a decided technological advantage and decides to use it to foment the American Revolution early.
- The 1994 novel Mysterium by Robert Charles Wilson is one of the creepiest examples of this trope, where a quantum experiment gone wrong flings the requisite American town into an alternate universe, where a particularly intolerant form of Gnostic Christianity is the dominant world religion. Because the environment is so strange, and the most innocuous act (like putting up Christmas decorations) can trigger harsh retribution, the townspeople are completely at a loss, though still somewhat Plucky(tm)
- The 1951 novel City at World's End by Edmond Hamilton: The town of Middletown (yes, that's what he called it!) is somehow flung to the far future, where the sun has expanded to a red giant and Earth is only barely inhabitable. The Middletowners find themselves at the mercy of their super-advanced descendants, who treat them with contempt. They're still Plucky Americans, though, whatever colour the sun is.
- Island in the Sea of Time and sequels by S.M. Stirling.
- Similar to, but not exactly the same as, more prosaic small-American-town-on-its-own tales like the TV series Jericho.
- Subverted in the Star Trek: The Original Series episode "Spectre of the Gun". While on the planet of the Melkots, Captain Kirk and his party find themselves in the town of Tombstone, Arizona in 1881. Kirk had to roleplay the shootout at the OK Corral. During the episode they learn that the entire town and its inhabitants were just an illusion.
- In GURPS Fantasy, humans came to the world of Yrth when a elven spell went wrong and teleported numerous small villages from medieval Europe, the Middle East and Japan to Yrth.
- Brigadoon is a small Scottish town that has this "showing up once every 100 years" thing.
- Chrono Cross: It happens to Chronopolis (it's transported 14300 years to the past), although it's debatable whether it could count as a town, it's more like a huge research facility.
- SCP Foundation: The SCP-110 entry consists of an underground city under the state of New York, with an area of approximately six square kilometers, that was transported to the past by an unknown entity imprisoned there.