How It All Goes Down
We start with some creepy witches cackling about some guy named "Macbeth," and then cut to post-battle, where we learn that this Macbeth has been kicking serious tail in battle—so much that King Duncan has decided to give him the title Thane of Cawdor.
Now it's time to meet Macbeth. He's prancing home on a dark and stormy night after defending King Duncan in battle with some skilled enemy-disemboweling. Understandably, he's feeling pretty good about himself. Just then, he and his good pal Banquo run into three bearded witches (the "weird sisters"), who rhymingly prophesy that Macbeth will be named (guess what?) Thane of Cawdor and King of Scotland. Just as Banquo is pouting about being left out, the witches tell him that he'll be father to a long line of future kings of Scotland.
The next thing we know, a guy named Ross shows up to say that, since the old Thane of Cawdor turned out to be a traitor and is about to have his head lopped off and displayed on a pike, Macbeth gets to take his place as Thane of Cawdor. Sweet! That takes care of the first prophecy. At this rate, the play will be over before lunch.
While Macbeth is waiting around for "chance" to come along and make him king, he starts getting restless. His ambitious wife, Lady Macbeth, prods him into acting like a "man" and killing King Duncan when the poor guy comes to Macbeth's castle for a friendly visit.
When Macduff (yeah, we know, there are more "Macsomebodies" in this play than an episode of Grey's Anatomy) finds the king's dead body, Macbeth kills the guards and conveniently accuses them of murdering the king. King Duncan's kids, Donalbain and Malcolm, find out what's happened, they high tail it out of Scotland so they can't be murdered too.
Macbeth is named king and things are gravy. Prophecies fulfilled! Except, wait. Macbeth starts to worry about the witch's prophecy that Banquo's heirs will be kings. Macbeth's not about to let someone bump him off the throne so, he hires some hit-men to take care of Banquo and his son, the unfortunately named Fleance. Banquo is murdered, but Fleance escapes.
Things go downhill for Macbeth, who's more haunted than an episode of Ghost Hunters. He pops in on the Weird Sisters for another prophesy, which comes in three parts: (1) watch out for Macduff; (2) No man born of woman is going to hurt him; and (3) Don't worry until Birnam Wood (a forest) moves to Dunsinane.
Macbeth breathes a sigh of relief with #2 and #3, since those are obviously impossible situations and mean that he's effectively safe. The one about Macduff has him a little worried, though, so he kills off Macduff's family. Naturally.
By now, people are starting to get a little suspicious. Macduff and Malcolm pay a visit to the awesome English king, Edward the Confessor, and start plotting with the English soldiers how to save Scotland from Macbeth's tyranny. Oh, and Lady Macbeth? She's not doing so hot. In fact, she basically dies of guilt. But Macbeth is safe, right? Not so fast. Macduff and Malcolm show up with their army and order troops to cut the branches from the trees in Birnam Wood for camouflage.
Remember what the weird sisters said about Birnam Wood moving to Dunsinane? Then you know where this is headed. Macduff corners Macbeth; calls him a "hell-hound"; tells him that he, Macduff, was "untimely ripped" from his mother's womb, i.e. delivered via C-section rather than being "born; and then cuts off his head. So much for the phony king of Scotland.