The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents. We live on a placid island of ignorance in the midst of black seas of infinity, and it was not meant that we should voyage far. The sciences, each straining in its own direction, have hitherto harmed us little; but some day the piecing together of dissociated knowledge will open up such terrifying vistas of reality, and of our frightful position therein, that we shall either go mad from the revelation or flee from the light into the peace and safety of a new dark age.
So begins one of the most influential stories in weird fiction and the seminal work of what would come to be known as the Cthulhu Mythos. The narrator of the tale is going through the papers of his great uncle, who died under circumstances that, at the time, weren't suspicious. However, as our protagonist pieces together the clues, he discovers that his uncle had stumbled on upon a terrible danger that has threatened the existence of mankind since the dawn of time and a horrible cult serves this evil.
"The Call of Cthulhu" is a world-spanning story that goes from the Arctic to the swamps of Louisiana to the uncharted reaches of the South Seas. Lovecraft's method is to slowly reveal the mystery through a collection of artifacts, newspaper articles, diaries and interviews that begins with a strange carving and an account of people having odd dreams and then progresses on to weirder and more horrifying episodes as each layer is peeled away. The result is that the reader is slowly drawn into this terrible, unfathomable universe where man has no place, yet where any who discovers this secret places his life in peril. All this builds to a climax set at what can only be described as an outpost of Hell; a place where all reason and morality vanishes into the Abyss.
As with most of Lovecraft, the prose can be a bit overdone and his protagonists seem as resilient as overcooked spaghetti, but if you're willing to buy into his conventions, the ride is worth the bargain.