The year is unknown. The place is somewhere on Earth. In Secret Cthulhu players are attempting to stop Cthulhu devouring the planet by enacting enough Rites of Salvation to protect the planet and all that currently live on it. Watch out though, there are secret Cultists among you, and one player is possessed by Cthulhu himself. These Cultists will be doing everything in their power to pave the way for Cthulhu, and the destruction and chaos he brings with him.
At the beginning of the game each player is assigned one of three roles: Cultist, Investigator, or Cthulhu. The Investigators have a majority, but they don’t know for sure who anyone is; Cultists must resort to what they know best to accomplish their goals: secrecy and sabotage. Cthulhu plays for the Cultists, and the Cultists know Cthulhu’s identity, but Cthulhu isn’t always aware of who his fellow Cultists are and must work to figure them out.
If that's not enough, you need to be ready for whatever the Chaos Deck throws your way.
—description from the designer