Final answer:
The speaker of the line is Hamlet, who seeks to confirm his uncle Claudius's guilt in his father's murder by observing his reaction to a staged play.
Step-by-step explanation:
The speaker of the line "The play's the thing / Wherein I'll catch the conscience of the king" is Hamlet, the protagonist of William Shakespeare's tragedy "Hamlet." In this line, Hamlet is devising a plan to confirm his uncle Claudius's guilt in the murder of Hamlet's father. By staging a play that mirrors the circumstances of the murder, Hamlet hopes to observe Claudius's reaction and determine whether he is indeed guilty. The Tragedy of Hamlet, Prince of Denmark, often shortened to Hamlet is a tragedy written by William Shakespeare sometime between 1599 and 1601.