As quoted from the Call of Cthulhu quick start game link
The simplest metaphor for a game of Call of Cthulhu can be likened to the fairy tale of the Little Dutch Boy. The dam had a crack and the Little Dutch Boy had to stand there with his finger in the hole to keep the water from flooding out and destroying the nearby town. However, instead of how the original tale played out, imagine that on the other side of that dam is a bloodthirsty shark, which is gnawing away at the Dutch Boy. He loses one finger, so he must put another one in. Then he loses another finger, and another. The hole is getting bigger, and he must stick his entire arm in, and the shark keeps biting. But if he fails, if he leaves his post, the dam will collapse, and many lives will be lost. And so he stays, resolute in his convictions. He may die, but humanity will live because of him.
You will be playing as members of a small Boston Private Investigation firm, hired to find out the real culprit of the Williams' murders, for the better or worse for your sanity. For things in Lovecraft’s world are much more than they appear, much more.
All you will need to play this game this http://www.chaosium.com/forms/coc_quick_start_bw.pdf, by following the Character creation rules, as I want this game to be played by both those who have played the game before and those who haven’t.