Horatio and Marcellus joined Barnado and Francisco found a ghost. The ghost that looks like the previous king, they think it is a warning. Horatio tries to talk to the ghost but it disappears. So Horatio decides they should tell Hamlet.
Ophelia
King Claudius tells everyone he is marrying Gertrude. Then Claudius and Gertrude question why Hamlet is so upset. Hamlet says that he wishes he could die due to marriage. Horatio, Marcellus, and Baranado tell Hamlet that they saw the ghost of his dad.
Ghost summons Hamlet
As Laertes prepares to leave and he warns his sister Ophelia to stay away from Hamlet.
Revenge
Lord Chamberlain gives Laertes a long piece of advice on how to act. He finally lets Laertes leave and then asks Ophelia what she and Laertes were talking about. Ophelia tells him about her love with Hamlet. Lord Chamberlain tells her he tells her that he forbids her from seeing him. She obeys.
On cold ramparts, Hamlet, Horatio, and Marcellus keep watch. Meanwhile a "party" is being held in the castle which Hamlet disapproves of. The ghost eventually shows and calls Hamlet to follow him.
The ghost claims to be King Hamlet and says that Claudius murdered him. The ghost commands that Hamlet gets revenge on Claudius for murder and for corrupting Gertrude. Hamlet promises to get revenge.