Hamlet's strongest theme is revenge seen from the side of the one who receives the order and is slow to carry it out: what to do with an accusation that comes from a ghost and that may be true or a trap. Around it turn pretense, madness, the corruption of the court, and death.
Hamlet receives the order in Act I, scene 5, and soon in Act II, scene 2 says the ghost "may be the devil" and that he will need "grounds more relative than this." He then stages The Mousetrap to see the king's reaction. When he finds Claudius praying, in Act III, scene 3, he spares him, and in the queen's chamber he kills Polonius, thinking it is the king.
The play places beside him two other sons: Laertes, who returns from France demanding his dead father and is himself the one who proposes the poison for the duel's sword, and Fortinbras, who marches with an army to take a piece of land that, in a captain's words, yields no profit "beyond the name." Seeing that army, in Act IV, scene 4, Hamlet censures himself for having a "father killed, a mother stained" and still letting everything sleep.
In Act V, scene 2, already with the proof of the royal dispatch in hand, he answers Horatio that being ready is all, and revenge is accomplished only in the duel, after he himself is wounded. Around this the play brings the distance between what one feels and what one shows (I have within me what goes beyond appearance), the rot of the kingdom (something is rotten in the state of Denmark), and death, with Yorick's skull.