What is "The Murder of Gonzago" in Hamlet, II.ii?
Hamlet tells the audience in act 2, scene 2 that he will have the players, who arrive at Elsinore in this scene, "Play something like the murder of my father / Before mine uncle." He intends, in other words, to put on a play that reenacts the circumstances of his father's murder (as described by the Ghost). He will then watch Claudius and judge from his reaction whether he is actually guilty of the murder before pursuing revenge. The Murder of Gonzago is the name of the play in question, intended to, as Hamlet says, "catch the conscience of the King."
The play is staged in act 3, scene 2, and Hamlet, just to be certain, asks Horatio to keep an eye on the King's reaction to events as well. When Gonzago is murdered in the play, the King rises in anger and makes his way quickly from the room. He is clearly shaken by the similarity between the action in the play and his actions. After witnessing his reaction, Hamlet tells Horatio that he takes the Ghost's word "for a thousand pound." He can be certain that his uncle really did murder his father. Tellingly, as the play is about to commence, Hamlet tells his uncle that its title is The Mousetrap. Hamlet indeed cleverly uses the play to "trap" his murderous uncle.
This is the play that Hamlet asks a traveling troupe of actors to perform. Rosencrantz and Guildenstern originally asked the troupe to perform at the castle, and Hamlet is delighted when he discovers they are coming. Hamlet adds some lines to the play to be spoken by the actors.
"The Murder of Gonzago" mirrors the events that have occurred in Denmark, specifically Claudius's murder of King Hamlet. The play is about a king who is poisoned by another man, but before the king dies, he talks with his wife about love, death, and remarriage. This scene is where Hamlet has asked the actors to add lines in the play that reflect what Hamlet what Hamlet wants to test about Claudius's regicide and his mother's betrayal of his father.
In "The Murder of Gonzago," the queen promises never to remarry, especially not someone who murders her husband, but the king says she will forget her promise once he is gone. Claudius takes offense at the scene and, when they get to the point where King Gonzago is poisoned, Claudius gets up and leaves. Hamlet determines he has proven that Claudius killed King hamlet, Hamlet's father.
This is the play within a play. Hamlet asks traveling actors to put on a play that is similar to the circumstances of his father's death. Hamlet also inserts lines into the original play to sharpen the focus on King Hamlet's murder. Hamlet and Horatio carefully watch King Claudius for any signs of recognition or guilt, recognition that Claudius does reveal. This is the proof that Hamlet needs to determine that the Ghost is an honest ghost and that Claudius did kill King Hamlet.