In the opening scene of the play, Marcellus explains to Barnardo that he has come with Horatio to witness the Ghost that mysteriously wanders in the night while they are guarding Elsinore Castle. The appearance of the Ghost has influenced Marcellus to inform Horatio of its existence, which is significant because Horatio is Prince Hamlet's close friend. Once Horatio witnesses the Ghost, who appears to be the recently deceased King Hamlet, he is motivated to tell Hamlet about the intriguing apparition. Once Horatio informs Hamlet about the Ghost's appearance, Hamlet stands outside the castle with Horatio and Marcellus and waits for the Ghost to appear in act 1, scene 4. In the final scene of act 1, Hamlet follows the Ghost and ends up discovering that it is indeed the spirit of his deceased father, who explains how Claudius assassinated him and demands that Hamlet avenge his death. Overall, the Ghost's initial appearance in the opening scene of the play drives the plot and influences Horatio to mention the Ghost's appearance to Hamlet. By the final scene of act 1, Hamlet has met the Ghost and vows to avenge his father's death by killing Claudius.

The appearance of any ghost in Shakespeare means that something very unnatural has taken place and the great chain of being is somehow off kilter. When the ghost appears to Marcellus and Horatio, they recognize the ghost as belonging to the former King Hamlet. Since Horatio is good friends with Hamlet, he knows that the person who should know about it first is the Prince. The appearance of the ghost sets the plot moving and provides very important back-story. Firstly, that King Hamlet was murdered in his garden, and secondly--it was done by Hamlet's uncle Claudius. This is earth shattering news to Hamlet since Claudius has recently married his mother.