Dress-Up Episode

More commonly (but not exclusively) seen in visual media like television and comics, the modern protagonists of the work in question suddenly and inexplicably need to dress as if it's the Victorian Era. It could be that they have to attend a Masquerade Ball without attracting attention to themselves, or that trace amounts of Time Travel have suddenly and inexplicably thrown them back to The Wild West. Maybe they have to establish diplomatic relations with an alien race that is convinced they live in the 1920s. Whatever the reason, the main characters of the work dress up in recognizable, elaborate or anachronistic costumes that they wouldn't otherwise wear.

Note that while examples of this trope are predominantly historical costumes, time travel is not a requisite. It's any time the main characters dress in costume for the duration of an episode/scene/whatever, usually in order to blend in or detract suspicion.

Is separate from, but may overlap, Halloween Episode. Related to Costume Porn and applicable to Going Native.

Anime and Manga

 * Done frequently in Ranma ½ to the point where it is practically a Running Gag and an Omnipresent Trope.
 * Yu-Gi-Oh: The first season episode "Evil Spirit of the Ring" had the main cast wearing the costumes of their favorite duel monsters--Yugi as the Dark Magician, Joey as Flame Swordsman, Tristan as Cyber Commando, and Tea as the Magician of Faith.

Film

 * In Rango, Rango and his posse masquerade as a traveling theater group in order to get close to the water thieves.

Literature

 * In the Discworld novel Jingo, Nobby, Colon and Vetinari disguise themselves as performers while infiltrating their current enemy city. Hilarity Ensues.

Live-Action TV

 * Happens in Star Trek on a regular basis.
 * In the Original Series episode A Piece of the Action, Kirk and Spock are sent to a planet whose citizens base their entire livelihood on the mob bosses of the 1920s (think Al Capone).
 * In the Next Generation, the Holodeck is often the setup for a Dress Up Episode.
 * Time travel is also a common cause of this.
 * Doctor Who. Since the entire premise of the show is based on time travel, this is pretty much a given.
 * In the episode "The Affair at the Victory Ball", Poirot and Hastings are invited to a fancy-dress (masquerade) ball and told to come as "someone famous". Poirot comes as himself.
 * Happens all the time on Supernatural.

Video Games

 * The Kasumi: Stolen Memory DLC for Mass Effect 2 opens with a formal party and offers the players a rare opportunity to see Commander Shepard out of uniform and Powered Armor.

Web Comics

 * Occurs occasionally in the webcomic Questionable Content at the bar that the characters frequent.