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 era in question. 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.
It is most commonly (but not exclusively) seen in visual media like television and comics, because it can have the greatest effect without cluttering the narrative. In literature, this can easily turn into a wordy Costume Porn.
Is separate from, but may overlap, Halloween Episode. Subtly different from Costumer, in which characters wear period costume in a dream sequence or alternate reality. Related to Costume Porn and applicable to Going Native. Compare Changed My Jumper.
- Chapter 112 of Kaguya-sama: Love Is War has Kaguya testing out multiple costumes in preparation for the cosplay cafe her class will be doing during the culture festival. She also uses it as a chance to get a rise out of Shirogane and force him to confess, but it fails because he had already decided to start being honest with his feelings for her in the previous chapter, which catches her completely off guard.
- 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.
- In Asterix the Legionary, Asterix and Obelix join up with the Roman Army and spend about half the book rockin' the uniform.
- A Nancy Drew graphic novel adaptation, "The Haunted Dollhouse," features the entire town of River Heights celebrating 'Nostalgia Week.' This means the entire town dresses, dances, and drives like it's the 1930's - Nancy even rents a vintage Roadster for the duration of the book.
- In Rango, Rango and his posse masquerade as a traveling theater group in order to get close to the water thieves.
- A season 6 episode of Castle requires the entire cast to dress and act like it's The '70s for most of the episode, in order to not confuse a witness who thinks it's still 1978.
- Doctor Who. Since the entire premise of the show is based on time travel, this is a given. Most of the time, however, they don't even bother with this, and the Doctor himself rarely changes his garb, but some episodes has the Doctor's companion dress up, such as when Rose dressed in '50s style clothing and Donna dressed like a flapper in the '20s. On one occasion, both Clara and the Doctor dressed to the nines, 1920s style, for a trip aboard a spaceship replica of the Orient Express.
- Legends of Tomorrow: The show lives on this trope. Practically every episode has the crew going to some different time period requiring a costume change.
- 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 in Star Trek on a regular basis.
- Time travel is a common cause of this.
- Star Trek: The Original Series: In the 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).
- Star Trek: The Next Generation: The Holodeck is often the setup for the main characters to dress up in outfits that are appropriate for the setting of the program.
- Star Trek: Picard: As part of their undercover op on Freecloud, Picard, Rios and Elnor wear costumes that are very different from their normal attire in "Stardust City Rag."
- Happens all the time on Supernatural. In one episode, "Frontierland", Dean wears a cheesy cowboy costume, and is made fun of because it looks nothing like how the people actually dress. However in another Time-Travel Episode to 1944, he lets someone from that time period dress him, and quite likes being a Badass in a Nice Suit.
- Power Rangers Time Force has the two-parter episode "Movie Madness", in which a mutant transports the Rangers to different movie genres, all with appropriate clothing (Trip and Wes as cowboys in a Western, Jen in a quipao in a Chinese martial arts film, etc.). When they're able to escape the movie dimension, even more dressing up occurs, complete with crossdressing and even an Actor Allusion to Vernon Wells' previous role in Mad Max 2: The Road Warrior.
- Dead Gorgeous: In "Reliving History", the school recreates life as it was 150 years ago, so the entire school is dressed in period garb.