The ghost of king hamlet speaks to hamlet, ha says that king Claudius has poisoned him. he tells prince hamlet that he wants his death to be avenged, however he must not hurt his mother.
Hamlet is in the castle plotting hs to gather information on claudiuis by having a play that shall reveal caudius's true colors
hamlet tells horatio what the ghost of his father told him. hamlet asks horatio to watch claudius's reaction when the play is going on. hamlet says if claudius displays no guilt during the play then the ghost was not his father
polonius is hiding behind the wall tapestry to listen to the conversation between Gertrude and hamlet to see why hamlet is upset, until he hears Gertrude scream during the conversation which leads him to scream causing hamlet to react by stabbing the tapestry thinking it was king Claudius.
claudius and laertes come up with a plan to ensure hamlets death, laertes shall fence and laertes shall have a sword with a poison tip and claudius will poison hamlets drink.
horatio and hamlet fight it out and lartes his hamlets with his sword injuring hamlet, laertes also gets injured by the poison tip sword meaning both of them will die. king claudius poisons hamlets drink just for gertrude to drink the drink killing her. this leaves claudius to get stabbed as well with the poison sword leaving only horatio left not dead.