An infamous play titled "The King in Yellow" has started in Arkham. The play, when read or viewed, has a mysterious power to unhinge the mind. But who would believe such a preposterous notion? It is up to investigators to uncover the truth behind the "King in Yellow", and to stop the dark powers that it may awaken.
Act cards are placed in a small deck in numerical order, starting with Act I on top and ending with Act III on the bottom. To play without this additional pressure on the investigators, simply remove the six "The Next Act Begins!" Mythos cards from the Mythos deck.
Each time one of the six Mythos cards entitled "The Next Act Begins!" is drawn and resolved, the top card of this Act deck enters play. While Act I and Act II each list a way for the investigators to prevent them from entering play, Act III cannot be stopped once it has begun. Should Act III ever enter play, an enormous charity performance of "The King in Yellow" takes place in Arkham, driving the townsfolk mad and destroying the city. In that case, the investigators immediately lose the game.