Wikipedia defines an ARG as such...

An alternate reality game (ARG), is an interactive narrative that uses the real world as a platform, often involving multiple media and game elements, to tell a story that may be affected by participants' ideas or actions.

The form is defined by intense player involvement with a story that takes place in real-time and evolves according to participants' responses, and characters that are actively controlled by the game's designers, as opposed to being controlled by artificial intelligence as in a computer or console video game. Players interact directly with characters in the game, solve plot-based challenges and puzzles, and often work together with a community to analyze the story and coordinate real-life and online activities. ARGs generally use multimedia, such as telephones, email and mail but rely on the Internet as the central binding medium.

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