The setting of Call of Cthulhu is a darker version of our world, based on H. P. Lovecraft's observation (from his short essay, Supernatural Horror in Literature) that "The oldest and strongest emotion of mankind is fear, and the strongest kind of fear is fear of the unknown." There are three primary eras of the original (BRP) game: the 1920s, the setting of many of Lovecraft's stories; the 1890s Gaslight supplements, a blend of occult and Holmesian mystery and mostly set in England; and modern conspiracy (Cthulhu Now). Recent additions include 1000 AD (Cthulhu: Dark Ages), 23rd Century (Cthulhu Rising) and Roman times (Cthulhu Invictus). The protagonists may also travel to places that are not of this earth, represented in the Dreamlands (which can be accessed through dreams as well as being physically connected to the earth), as well as travel to other planets or the voids of space.
The players take the roles of ordinary people drawn into the realm of the mysterious: detectives, criminals, scholars, artists, war veterans, etc. Often, happenings begin innocently enough, until more and more of the workings behind the scenes are revealed. As the characters learn more of the true horrors of the world and the irrelevance of humanity, their sanity (represented by "Sanity Points", abbreviated SAN) inevitably withers away. The game includes a mechanism for determining how damaged a character's sanity is at any given point; encountering the horrific beings usually triggers a loss of SAN points. To gain the tools they need to defeat the horrors – mystic knowledge and magic – the characters may end up losing some of their sanity, though other means such as pure firepower or simply outsmarting one's opponents also exist.
Call of Cthulhu has a reputation as a game in which it is quite common for a player character to die in gruesome circumstances or end up in a mental institution. Unlike in most other role-playing games, eventual triumph of the players is not assumed.
Call of Cthulhu uses the Basic Role-Playing system used by other Chaosium games (first seen in RuneQuest). For as long as they stay functionally healthy and sane, characters grow and develop. Call of Cthulhu does not use levels, but is completely skill-based, with player characters getting better with their skills by succeeding at them. They do not, however, gain "hit points" and do not become significantly harder to kill.