If you saw a ghost in the form of your dead father, would you believe what it said or what it told you to do? This is one problem Hamlet had to face. This ghost said he was King Hamlet Hamlet's father. He told Hamlet some shocking information, that he was murdered by his brother and is living in everlasting hell, until Hamlet would avenge his fathers death. Hamlet has to find out if this is the truth, and if so, the question is how to kill and where. "Rest, rest perturbed spirit." Hamlet has to figure out what is true and what is not, he does know that he has to find the truth out, proof of what the ghost told him. "Murder most foul, as in the best it is, but this most strange and unnatural." This makes sense, the kings brother kills him, takes the throne, and marries his wife the queen, but Hamlet wants / needs proof.
Hamlet has figured out quite a cleaver plan to get proof that King Claudius killed King Hamlet, a play. "The play's the thing wherein I'll catch the conscience of the king." Hamlet has arranged a reenactment of the murder in the form of a play. He believes once the king knows Hamlet knows he will act out in a way that shows his guilt, or he will do nothing and that will prove his innocence. The king reacted, the ghost was being truthful. Hamlet now has to begin to plot against the king to avenge his fathers death. Claudius realizes that Hamlet knows and begins to piece together what might happen, and starts to think up ways to kill Hamlet.
The ghost reappears to keep Hamlet on the right track.