Warning: Spoilers follow
The Call of Cthulhu is one of H. P. Lovecraft's best known short stories. It is the only story in which Cthulhu himself makes an appearance. It is a story composed of two major narratives linked by a third, the voice of the "author". Only the author pieces together the whole truth and significance of the information he has in his possession, and he recounts it to the audience in the way that he discovered it.
It begins with an account of the death of a prominent professor at Miskatonic University and a study of his papers. These papers include the account of a raid on a supposed Satanic cult. A study of the cultists brings to light clues about the hideous creature that they worship, Cthulhu. The Cthulhu creature, said to have come with his alien followers from the stars millions of years before the dawn of Man, now rest in a death-like sleep in their sunken city of R'lyeh.
The second part of the story picks up here with the recounting of the log of a first mate on a ship that discovers the sunken city, now risen to the surface of the Pacific Ocean. The city has resurfaced because "the stars are right" and the time for the awakening of Cthulhu and his spawn is at hand. Cthulhu emerges from his tomb and almost all of the crewmen are slain. But the stars are not quite right, after all, and Cthulhu and R'lyeh return to the deep after the first mate makes a harrowing escape. The story concludes with the death of the first mate under mysterious circumstances, and the author's fear that he, too, will perish by the hands of the Cthulhu cultists.
There is also a role-playing game based on the Cthulhu Mythos called Call of Cthulhu.