The game takes place over four rounds. The Investigators must uncover all the Elder Signs to win; the Cultists win when Cthulhu is revealed or if the game ends before all the Elder Signs are discovered.
Before each round players receive a number of Investigation cards, which they look at but then shuffle and put in front of themselves so they know what cards they have, but they don't know which card is which. Each round has a number of actions equal to the number of players in the game. Players take actions to reveal Investigation cards, and gather all unrevealed cards are shuffled an redistributed evenly among the players.
Reveal Cthulhu, and the Cultists win instantly. Reveal all the Elder Signs, and the Investigators win. If you want to play multiple rounds (it's highly recommended), the losers in each game get Insanity tokens. Get three tokens, and the night is over with the winner(s) being those most sane.