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.