In Act III of Macbeth, Banquo and Macbeth discuss the witches' prophecy. Banquo notes that Macbeth's prophecies have become reality. Banquo also is intrigued about the prospects of his own prophecy (that his son/s will be king). Macbeth reminds Banquo about the banquet. Then, Macbeth, also worried about Banquo's prophecy, hires men to kill Banquo and his son Fleance. Banquo is killed. Fleance escapes.
Macbeth discovers that Banquo is killed but Fleance escaped. At the banquet, plagued by guilt and fear, Macbeth sees Banquo's ghost. Lady Macbeth ends the party for fear that Macbeth's actions and/or words might reveal his guilt.
Hecate scolds the other witches for not including her in their dealings with Macbeth. She promises to cast a spell that will lead Macbeth further along the path to self-destruction.
Lennox suggests (to a Lord) that Fleance did not kill Banquo and Duncan was not killed by Malcolm and Donalbain, which is the general consensus of the other players. Lennox, on the other hand, suspects Macbeth has had a hand in both murders.
In Act IV, the witches give Macbeth three more warnings and a vision. The three warnings are: beware of Macduff, no man of woman born can harm Macbeth, and Macbeth shall never be killed until the Great Birnam Wood (forest) comes to Dunsinane hill to fight him. The vision is of a procession of kings, the last holding a reflection of Banquo. Lennox approaches, saying he saw none of this. Macbeth reveals (as an aside) his plans to kill Macduff's family.
Lady Macduff is very upset that Macduff has fled to England. She is so angry that she tells her son that his father is dead. Her son does not accept this. A messenger warns Lady Macduff to leave. She is obstinate and will not leave. Murderers arrive and kill her and her son.
In England, Malcolm questions Macduff to make sure he can trust him. Malcolm, in despair, supposes that he would make a worse king than Macbeth. Macduff tells Malcolm that he (Malcolm) is the rightful heir. They agree that Macbeth must be stopped. Ross enters and tells them Macduff's family has been killed. Macduff vows revenge.