On the ramparts of the Danish castle Elsinore, the guardsman Barnardo relieves Francisco. The men are nervous, calling out "Who's there?" Marcellus, another guard, and Horatio, a nobleman, arrive.
Nervous cries of "who's there?" builds dread and develops theme of uncertain reality.
A Ghost appears. It looks like the recently deceased Old Hamlet, King of Denmark. Horatio tries to speak to it, but it disappears.
The appearance of the ghost confirms something is not right in Denmark.
Horatio says the ghost might be warning of an attack. After all, the prince of Norway, Fortinbras, is raising an army to retake lands that Old Hamlet won in battle from Fortinbras' father.
The ghost is connected immediately to the theme of revenge—Fortinbras's revenge.
The Ghost reappears but disappears again without speaking when the cock crows to greet the dawn. Horatio decides they should tell Hamlet, the dead King's son, about the ghost.
Every father/son relationship in the play leads to revenge.