Although Macbeth is a fairly straightfoward play, with few real subplots, there are several crucial scenes.
The first would be, obviously, Act I Scene 3, when Macbeth and Banquo encounter the witches. It is there that Macbeth has his ambition piqued by the witches' prophecy that he will be Thane of Cawdor and "King hereafter." It is this event that sets the plot in motion. Yet it is not a given that Macbeth is going to carry out the murder of Duncan. In Act I Scene 7, he vacillates over committing the murder before ultimately deciding to call off the plot. Lady Macbeth, however, goads him into the act, questioning his manhood and his honor. Later, the witches again figure heavily in the plot when, in Act IV Scene 1, they summon three visions for Macbeth that convince him of his own invulnerability to his enemies. They tell him that he can only be destroyed when Birnam Wood marches on Dunsinane (his castle) and then only by a man who was not born of a woman. Another crucial scene is Act IV Scene 3, in which Duncan and Macbeth agree to overthrow the tyrannical Macbeth. Finally, there are two crucial scenes in the Act V. In Scene 5, Macbeth discovers both that his wife has committed suicide and that, apparently, Birnam Wood is advancing against the castle. In the final scene, Macbeth discovers that Macduff was in fact born by Caesarian section, and thus not "of woman," and realizes that the witches have tricked him. He dies, and Malcolm becomes king.