A mysterious note from a stranger changes the plot in some way. Generally, it is shoved under or into something such as a locker, postbox, or a door. If the recipient sees the note drop on his mat and dives for the door, he will occasionally catch the messenger in the act of posting the note; however, the messenger usually leaves so fast it's as if he teleported.

Can be anything at all, for example:
* A clue from an anonymous source
* A note from a past or future self
* A revealing tape of some kind
* A threat of some kind
* Ransom demand
* Invitation
* etc

Not to be confused with BrownNote or any other sound related trope, although it can contain, or be a recording of any kind.
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!!Examples:

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[[folder: Anime & Manga ]]

* In ''Manga/OresamaTeacher'', both Mafuyu and Hayasaka receive notes in their lockers saying "360 degree warning". This presages the appearance of a new character, a wacky fake 'ninja' who is trying to destroy the Public Morals Club.

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[[folder: Fanfiction ]]

* So many times it's not even funny in ''HarryPotterAndTheMethodsOfRationality''

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[[folder: Film ]]

* ''MurderByDeath''. While Sam Diamond and Miss Skeffington are in their room, they hear a ticking sound. A note is slipped under their door that says, "That ticking sound is a bomb that will go off in 30 seconds. Signed, The Murderer."
* ''TheIncredibles''. The "note" is in the form of a digital tablet with a video message inviting Mr. Incredible to do secret superhero work.
* In ''{{Unsane}}'', a horror/thriller author receives one about a series of murders related to his latest book.
* ''Film/{{Memento}}'': The hero receives a few of these. Some of them from himself, since he suffers from anterograde amnesia.

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[[folder: Literature ]]

* ''Literature/{{Home}}'': Frank Money receives a note that says, "Come fast. She be dead if you tarry." This causes Frank to start his journey home to save his sister.
* In ''[[SherlockHolmes The Hound of the Baskervilles]]'', Sir Henry Baskerville receives a message consisting of words cut and pasted from the ''London Times'' warning him to stay away from the moor (where Baskerville Manor was).
* Mysterious poison-pen letters (together with pranks and outright vandalism) are part of a plot against Shrewsbury College, Oxford in ''[[LordPeterWimsey Gaudy Night]]''.
* A couple of these appear in ''[[AuntDimity Aunt Dimity Takes a Holiday]]''. The notes' author employs the classic pasted-letters-cut-from-printed-material technique.

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[[folder: Live Action TV ]]

* In ''Series/TheXFiles'', Agent Mulder repeatedly receives revealing documents on various conspiracies from his informants. They try to be sneaky about it.
* In an episode of ''Magic City'', an envelope is shoved under the door. When opened, it contains a photo with "$20,000" written on the back. The photo shows one of the heroes in bed with a mobster's wife and is clearly an attempt at blackmail.

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[[folder: Pinball ]]

* In ''[[Pinball/WHODunnit WHO dunnit]]'', HardboiledDetective Nick Spade receives an unsigned note asking him to meet someone in the basement of Tony's Palace. It's actually a setup for an ambush by the murderer.

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[[folder: Video Games ]]

* SilentHill4: Henry can receive messages from the dark world if he slides blank pieces of paper under his door. [[ShameIfSomethingHappened Better go check on your neighbor!]]
* VideoGame/TheDreamMachine has it's main plot started by one Viktor finds in his new apartment.
* {{World of Warcraft}}: At the start of the Mists of Pandaria legendary questline, you receive this [[http://www.wowhead.com/item=87261/mysterious-note conveniently named item]], indicating that the last of the Black Dragons wishes to recruit you to save the world.