The three witches meet Macbeth and Banquo and prophecies that Macbeth will be king. This causes Macbeth to think to become king he has to kill the current king (Duncan).
Lady Macbeth finds out about the prophecies and tells Macbeth he must kill the king. So she makes a plan.
Macbeth kills the king and Lady Macbeth drugs the chamberlains and puts blood on them to make it seem like they did it. Macduff finds the dead body and Duncan's sons Malcolm and Donalbain fear for their lives so they flee the country.
Since the sons are not there to take over the thrown. Macbeth is next in line and becomes king.
Macbeth thinks his best friend Banquo knows about the murder so he hires murderers to kill Banquo and his son Fleance, but Fleance escape.