Why is the ghost in Act One of Shakespeare's Hamlet important?
The ghost that appears in Act One of William Shakespeare's Hamlet is the critical catalyst to the action of the remainder of the play. The ghost--which first appears to Bernardo, Marcellus, and Horatio on the walls of Elsinore--does not speak, but rather wanders in and out of sight. The men tell Prince Hamlet of this strange occurrence, and they gather once more on the rampart to see if the ghost will show itself.
This time, the ghost beckons for Hamlet to follow him. Despite the protestation of the sentries and Horatio, Hamlet does just that. When Hamlet urges the ghost to speak because he is "bound to hear," the ghost replies, "So art thou to revenge, when thou shalt hear." In fact, the ghost is carrying a terrifying message, which we can see in his speech below:
I am thy father's spirit
Doom'd for a certain term to walk the night,
And for the day confined to fast in fires,
Till the foul crimes done in my days of nature
Are burnt and purged away...
The ghost asks Hamlet to "[r]evenge his foul and most unnatural murder," revealing that "[t]he serpent that did sting thy father's life / Now wears his crown." In other words, King Claudius (Hamlet's uncle, who has now married Hamlet's mother and assumed his position on the throne) was the one to betray and murder Hamlet's father, the late King Hamlet. Hamlet is called to action to avenge this injustice, and after asking his friends to "[n]ever make known what you have seen to-night," he sets off to make good on his father's request. It is this journey for vengeance that drives the rest of the play.