Banquo walks to Macbeth's castles and discusses his "cursed thoughts" Macbeth denies thinking anything of it. They agree to talk at a later time and Banquo leaves. Right after, Macbeth has a vision of a bloody dagger that is pointed at Duncan.
Even though having seconds thoughts about murdering him, Lady Macbeth rings the bell as a signal and Macbeth proceeds to Duncan's chamber to murder him but forgets to leaves the daggers with the chamberlains.
Duncan is found dead. Lady Macbeth faints and Duncan's sons flee. Macbeth claims to have killed the chamberlains to prove loyalty to Duncan.
Ross and an old man talk about the weird things happening over the past few days with animals eating each other around the kings death. they suspect the sons are at fault because they fled. Macbeth is crowned king.