I don't know about you, but when I think of King John, I think of him:

but Shakespeare managed to write a play about King John without bringing up either Robin Hood or the Magna Carta. Since he's the genius playwright I don't suppose we can fault him too much, but it does seem to me that he wasted an awful good opportunity, there.
Shakespeare's King John is a lot more like him:

and fans of The Lion in Winter will also recognize Queen Elinor and King Philip. Fans of cinema in general will note that King John has the distinction of being the first Shakespeare play ever filmed.
The Life and Death of King John shares much of its plot with the earlier, anonymous The Troublesome Reign of John, King of England, but nobody's certain if it's an earlier draft, also written by Shakespeare, or whether there's another, even earlier play that they both are sourced from, or if Shakespeare was just a big ol' copycat. In any case, what is certain is that the King John of the play has a lot in common with Queen Elizabeth: at odds with another, possibly more legitimate claimant to the throne, at odds with the Pope, there's even an incredible sinking armada.
Elizabeth, however, made much better choices. The play begins with Elinor and John receiving word that King Philip of France is acknowledging Arthur (Geoffrey (John's older brother)'s son) the true King of England and about to wage war on his behalf. After promising to do likewise to defend his throne, John hears the case of Philip and Robert Faulconbridge. Philip, the older of the two brothers, has been disinherited by his father, and Robert is claiming all his land, on account of everyone thinks he (Philip) is a bastard. And, in fact, he is, but it soon becomes clear that he's the bastard of one Richard. You know, the Lionhearted? So, while Robert gets the land, Philip gets a promotion, and a new name (Richard Plantagenet).
Faulconbridge (who is, by the way, henceforth refered to in the script at Bast.), is pretty much everyone's favorite character. He's funny, he's loyal, he's a good man in a fight, and he's more or less the most honorable person in the play.
Anyway, off they go to France, where they encounter King Philip, Arthur, Constance (Arthur's mother), and the Duke of Austria (the guy who killed King Richard) at the gates of Angiers. After some wrangling between Elinor and Constance, the assorted nobles ask the men of Angiers to say who they think is the rightful king, to which the smartypants townsfolk reply, "We'll tell you when you're done fighting." So the two armies fight for a while, but then Faulconbridge suggests that the two armies should join forces to sack Angiers, on account of their indecisiveness, and then continue on with their present conflict. To which the Angiersfolk reply, "Oh, you mean this gate key?" Okay, not really, but they immediately come up with the idea of marrying Blanche of Spain (John's niece) to the Dauphin of France, in order to come to some kind of accord that doesn't end with their town being burnt to the ground. Fortunately, for them, everyone (except Constance and Arthur) goes for it, and the wedding's on.
Unfortunately, just then Cardinal Pandulph shows up to ruin everyone's fun by reminding them that John has been thwarting the Pope's will and is due to be excommunicated with extreme predjudice. He convinces Phillip to make war once more on England, which gives Faulconbridge the opportunity to kill the Duke of Austria, who he's been almost constantly (and entertainingly) baiting this whole time. John says, "Screw this, we're outta here," and heads back to England, but not before taking Arthur captive. At which point this exchange happens, and I'm quoting it here so you can refer back to it later.
K. John: He is a very serpent in my way,
And wheresoe'er this foot of mine doth tread,
He lies before me. Dost thou understand me?
Thou art his keeper.
Hub.: And I'll keep him so,
That he shall not offend your Majesty.
K. John: Death.
Hub.: My lord?
K. John.: A grave.
Hub. He shall not live.
Arthur's capture gives Constance a chance to chew the scenery, which she especially loves to do, but, of course, Shakespeare being the genius playwright, manages to still give her her humanity. Just about when you've gotten sick of her handwringing, hair-tearing histrionics (and so, by the way, has everyone on stage), she demonstrates that her melodramatic tendencies don't entirely preclude actual depth of feeling:
K. Phi.: You are as fond of grief as of your child.
Const.: Grief fills the room up of my absent child,
Lies in his bed, walks up and down with me,
Puts on his pretty looks, repeats his words,
Remembers me of all his gracious parts,
Stuffs out his vacant garments with his form;
Then, have I reason to be fond of grief?
And, it seems she has reason to fear, because the next we see of Arthur, old Hubert's fixing to put out his eyes with a hot poker. In a harrowing scene, Arthur manages to convince Hubert not to blind or kill him.
Which is a good thing, because King John is having trouble with some of his nobles, who think that not only should Arthur not be killed, but that he shouldn't be imprisoned, either (eye-gouging is right out). At which point Hubert helpfully shows up to inform the King that he's discharged his duty and killed Arthur. Oops.
K. John: Why urgest thou so oft young Arthur's death?
Thy hand hath murd'red him. I had a mighty cause
To wish him dead, but thou hadst none to kill him.
Hub.: No had, my lord? Why, did you not provoke me?
K. John: It is the curse of kings to be attended
By slaves that take their humours for a warrant
Fortunately for the King, Hubert goes on to confess that he didn't actually kill Arthur after all. John sends him running after the nobles to tell them the good news.
Meanwhile, back at Hubert's house, Arthur throws himself off the walls and dies. The nobles come across his body in the streets, and, soon after, Hubert catches up to tell them he didn't actually kill Arthur after all. Hilarity ensues.
The nobles go join the Dauphin's invading armies. In order to stem the invasion/rebellion, John makes up to the Pope, and Pandulph tries to convince France to back off, but to no avail. Just when things are looking bleak, the French armada sinks somewhere in the Channel, and the fickle English nobles, upon hearing that the Dauphin plans to kill them once he's conquered England, defect again, leaving Louis to run home to Paris.
Unfortunately, John has been poisoned by a disgruntled monk, and can't savor the victory. His description of dying is painfully poetic: "I am a scribbled form, drawn with a pen/Upon a parchment, and against this fire/Do I shrink up." His son Henry arrives to pick up the pieces, and the Bastard Faulconbridge is loyal to the last lines of the play, which are all about how England will never be conquered from without as long as it manages not to be undone from within.