The ghost of Hamlet's father appears and tells Hamlet to avenge his death by killing his uncle, the new king, but not to harm his mother. Hamlet swears revenge but then begins to doubt if the ghost is truly his father or an evil spirit trying to mislead him. To get proof of his uncle's guilt, Hamlet decides to put on an act of madness and include a play within a play depicting his father's death, hoping his uncle's reaction will confirm he is the murderer.