The appearance of the ghost to Hamlet is what starts everything going. The killing of Polonius is another key event. Most incidents in the play conspire to keep the plot from moving.
Claudius and Laertes plot to kill Hamlet.
The beginning plot of Hamlet ended in the last scene of Act 5.
Disney based the Lion King loosely around the plot of Hamlet- missing some vital plot points nevertheless.
Hamlet is about a young man (Hamlet) who is told by the ghost of his father that Hamlet's uncle and stepfather murdered him. Hamlet acts crazy while waiting for the right moment to kill his uncle, but his uncle is spying on him and guesses the truth. In the end the uncle's plot to kill Hamlet almost succeeds, but before he dies, Hamlet completes his revenge.
Dude slept with my mom.
The main plot is the revenge plot. Hamlet learns his father was killed by his uncle; he pretends to be crazy to get close to Claudius and does eventually kill him. There is a subplot about Ophelia. There is another subplot about his relationship with his mother. There is yet another about his relationship with Rosecrantz and Guildenstern.
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If we take it that Hamlet killed Polonius believing him to be the king, and therefore as a part of his plot for revenge, then Polonius's death, and indirectly Ophelia's arise from his act of revenge. Claudius of course dies from Hamlet's revenge. Gertrude, Laertes and Hamlet die as a result of Laertes and Claudius's plot. Rosencrantz and Guildenstern die just because Hamlet doesn't much like them.
Hamlet is told by the ghost of his father that he was murdered by Hamlet's uncle Claudius who is now the king and Hamlet's stepfather. In the process of pursuing his revenge, Hamlet kills Polonius, Claudius's counsellor and the father of Hamlet's girlfriend Ophelia. Ophelia goes mad and dies, and her brother Laertes comes vowing vengeance for his father's death. Laertes and Claudius plot Hamlet's death, but although Hamlet is mortally wounded, their plot misfires, killing Hamlet's mother and Laertes. Hamlet kills Claudius before he, too, expires.
The play within the play - the enactment by the players of "The Murder of Gonzago," with additions by Hamlet.
The incident in which Hamlet gets off the ship to England plays an important role in changing the entire plot. As he was being "escorted" to England by Guildenstern and Rosencrantz, he switches the letter that the King had given with the one he writes. The original message that the King had wanted to pass on contained an order to kill Hamlet in England. However, Hamlet switches the letter with a new one which orders for Rosencrantz and Guildenstern to be executed. After this, a pirate ship attacks the ship that Hamlet was on. We are not given much detail on such an occurrence but we find out that Hamlet got onto the pirate's ship as the pirates were attacking the ship that was supposed to take Hamlet to England. Shakespeare does this in order to bring Hamlet back to Denmark and resume the plot. Although, the literary merit behind such a plot manipulation is widely debated as being either acceptable or flagrantly unacceptable.
They serve a more serious purpose. They are used by the King to spy on Hamlet, that's hardly comic relief. (Even though their fate was funny and well-deserved)