In William Shakespeare's Hamlet, the titular character believes that Claudius, his uncle and the current king of Denmark, murdered his father, who was Claudius's brother and the former king of Denmark.
Although Hamlet has strong suspicions and has been informed of this by the ghost of his father, he does not have any proof. Conflicted and not wanting to act based on speculation alone, he develops a plan to help him determine whether or not his uncle is guilty.
Hamlet learns that a group of actors will be visiting the castle, and he sees an opportunity. He asks the actors to perform The Murder of Gonzago, the plot of which is similar to events surrounding his father's death. He writes an additional scene and asks the actors to incorporate it into the play. Hamlet plans to observe Claudius's reaction to the play, and to the added scene in particular. He asks his close friend, Horatio, to help him observe Claudius during the performance. He believes that Claudius's reaction to the play will reveal whether or not he is guilty of killing his father.
The play depicts a king and queen, seemingly in love. A man, later revealed to be the king's nephew, pours poison into the king's ear, killing him in his sleep. The man then successfully attempts to seduce the queen.
During the scene in which the king is killed, Claudius becomes suddenly perturbed. He orders that the torches be lit and abruptly exits the room.
Hamlet and Horatio both agree that Claudius's reaction is extreme and strongly suggestive of guilt.