The theme of deception has a strong presence in Act I of Hamlet in two distinct ways: through the ghost’s revelation about Claudius and through Hamlet’s decision to act crazy.
When the ghost appears to Hamlet, the ghost reveals himself as the ghost of Hamlet’s father. Hamlet’s father was murdered (which leads to Hamlet’s melancholy mood in the early parts of the play). During the ghost and Hamlet's first meeting, the ghost reveals he was murdered by his brother Claudius. The ghost claims Claudius poured a poison directly into his ear canal. Claudius is now Denmark's king and married Hamlet’s mother. If the ghost is telling the truth, Claudius has deceived the entire country.
In his effort to figure out the real reasoning behind the ghost’s story, Hamlet decides to act crazy or to put an “antic disposition on.” Now it is Hamlet who is being deceptive. Of course, Hamlet’s actions after this decision have always made scholars wonder whether Hamlet is only acting crazy or if Hamlet actually goes insane.