The ghost tells Hamlet that he is, in fact, the ghost of his dead father. And there’s more: the ghost claims that Claudius killed him, taking his throne and his wife in the process. He wants Hamlet to kill Claudius in revenge. Shocked, Hamlet agrees and vows to avenge his father’s death. When Horatio and Marcellus reenter, having followed Hamlet to the scene of his chat with the ghost, they promise to keep quiet about what the ghost told Hamlet. Hamlet also tells them that he might start acting a little crazy because he’ll be using insanity as a cover while he investigates what’s really going on.