Claudius announces that Laertes and Hamlet should fight a duel. Before they do so, Claudius poisons the tip of Laerte's sword, and, poisons Hamlet's wine! Laertes wounds Hamlet, but in the confusion, they lose their swords, and Hamlet picks up the poisoned sword by mistake, managing to wound Laertes with it. Gertrude drinks from the poisoned cup, the dying Laertes reveals Claudius's dastardly plan, and Hamlet finally stabs Claudius, before collapsing, just as Fortinbras' army march in! (Hope this helps)!