The ghost of King Hamlet is the one who sets the action for the rest of the play. The ghost appears in the first scene of the play letting the audience know that the story is going to have a supernatural element and creating a question in the minds of the audience because we are not told why this ghost is appearing. We are told that Denmark is preparing for war, but that still does not seem to fully explain the appearance of the ghost. Next we find out that Hamlet is upset that his mother remarried so soon after his father's death and that she married her brother-in-law. Still, none of this gives the audience an understanding of why the ghost appears. When Hamlet speaks to the ghost, he is at first both horrified and curious. His dead father wishes to speak to him after all. Then when the ghost says that his own brother, Hamlet's mother's new husband, murdered him and that he wants Hamlet to avenge this murder, the audience knows what the main plot is going to be - Hamlet carrying out his father's wishes. This information is finally given to us in the last scene of the first act. The ghost is essential. If Hamlet had simply uncovered the murder of his father through investigation, we would not have the same impact and mysteriousness that we do with the news coming from the murder victim himself. The ghost also helps to explain Hamlet's sluggishness toward action. He finally decides at the end of Act 2, to use the play to determine whether or not the ghost was telling the truth. This lets us know that he's been doubting what the ghost said, or if he even really saw the spirit of his father. The ghost's reappearance at the end of Act 3, further establishes the fact that Hamlet has not acted yet on his father's wishes, and reminds Hamlet, and the audience, that Gertrude is not to be included in Hamlet's quest for revenge. Without the ghost, we would simply have a detective story with an element of revenge in it. With the ghost, we have the question surrounding the veracity of the ghost, why it only speaks to Hamlet and is not even visible to Gertrude, and the spookiness of a ghost itself.