The opening scene of Hamlet sets up the play in the genre of revenge tragedy and is one of uncertainty, fear, and foreboding. The time is midnight. It is cold and dark. The presence of two sentries suggest a potential for tension or conflict; sentries are not needed in a peaceful environment but only in one where worries about war or crime are in the foreground. Since only one sentry stands on the tower at a time—the only point at which two would meet is a change of shift—we have a sense of isolation in the dark in a tower far removed from the social life of the castle below.

Marcellus and Horatio appear and introduce a discussion of the apparition, adding an element of supernatural horror which intensifies when the Ghost appears. A further theme of isolation is expressed by the Ghost that simply appears in silence and leaves without speaking to anyone. The Ghost is also isolated. The final discussion which explains the backstory concerning the dispute between King Hamlet and Fortinbras adds to the sense of tension with potential for violence. Horatio's introduction of the topic of Julius Caesar and bad omens further intensifies the suspense and sense that something horrible is about to happen.

The tone in the opening scene of the play is mysterious. It begins with one guard challenging the approaching person so that immediately the audience gets set up for a story with lots of drama and action.  In l. 25, Horatio asks if the "thing" has appeared tonight.  The audience does not know yet what the "thing" is to which he refers, and the audience doesn't find out what Horatio, Marcellus, and Bernardo are talking about until the ghost itself appears.  Shakespeare creates the tone of mystery by not naming the "thing", but just letting it appear.  Add to that the fact that the ghost appears once again in the first scene and it never speaks. Also, the question of Denmark's war preparations creates this air of mystery.  The audience knows nothing yet about why the ghost appears, about what its importance is, and they know very little about who the ghost is the spirit of or the political situation in Denmark.