Machbeth and Banquo were returning from a battle when they encountered three witches. The witches said that Machbeth would be titled Thane of Cawdor and then become king. They also said Banquo's children would become royals
The witches disappeared. They stopped at a camp, where messengers told them Machbeth was now thane of Cawdor, making him believe the prophecy was true and he would become king. He told his wife, Lady Machbeth, in a letter.
Lady Macbeth is excited to become queen, however she wants to do it fast through killing the current King Duncan. She fears that Macbeth is too much of a loyal knight to kill his own king, so she begins to conspire
Duncan arrives at their castle for a feast. Lady Macbeth sees this as the perfect time to kill him, but it takes her some time to convince Macbeth to kill him. Eventually he agrees and they kill him, not being found out for their treason.
MacDuff arrives at the castle, discovering Duncan's corpse. This causes Duncan's sons, who would become kings, to flee to England because they feared that they would be suspected of the murder.
Macbeth is crowned king of Scotland, as Duncan and his sons were no longer in the picture the throne went directly to Macbeth and Lady Macbeth.