I was a big fan of Lovecraft, I read all of his books and short stories at one point or another. They all are some take or another on his 'Cthulhu Mythos', basically a mythos of supernatural gods and creatures. The game is not about a specific story (though as the previous poster stated, it has elements of several ), its an original story set in the Lovecraft universe and I think is a very good adaption of the mythos and its ideas. Call of Cthulhu is just one of Lovecraft's many stories.
Incidentally, Lovecraft himself never used the term 'Cthulhu mythos', nor was Cthulhu the 'main' god out of all the gods in his stories. It was some time later August Derleth coined the term, and subsequently Call of Cthulhu became the most famous of Lovecraft's stories.