I was drawn to reading "King John" because of Claire McEachern, who wrote the introduction. In case you don't know, Ms McEachern is a Shakespeare scholar and professor at UCLA who has written several intros for The Pelican Shakespeare series. Her insight into Prince Hal ("Henry IV, Pt. 1") is the most insightful I have read, and got me reading her other introductions, including the one for "King John." Thanks to her, I read the play, which is rarely performed today, but is in fact top-fight Shakespeare, with a cast of memorable characters, memorable lines, and a page-turner to boot.
This is the same King John who was forced into signing the Magna Charta, which Shakespeare left out of the play. Why? Apparently, Queen Elizabeth did not want to be reminded of it, and being politically astute the Bard was not going to be the one to remind her. For the record, King John reigned from 1199 to 1216.
As with history, so with Shakespeare's play, King John is not an admirable character. He's a snake, a political expedient who plays Rome to his own advantage, gets fearless Richard Plantagenet (a.k.a. "the Bastard") to lead his army, and who is not above having his rivals for the throne put to death.
The play revolves around young Arthur, rightful heir to the throne that John has so ignobly usurped. King Philip of France supports Arthur's claim and threatens an invasion. John invades France first and the result is a comedy of errors revolving around both armies and the town of Angiers in France. The looming battle is resolved by the marriage of Blanche, niece to King John, and Lewis, Dauphin of France. John then has Arthur imprisoned, which leads to another war between England and France, and allows Shakespeare the opportunity to create yet another of his great female characters, Constance, mother of Arthur. Her lines alone makes "King John" worthwhile.
Another of the great Shakespearean characters is the Bastard, Sir Richard Plantagenet. He's the type Shakespeare seems to relish--forthright, brutally honest, funny, irreverent, and truly a brave heart. Think actor Richard Burton in "The Taming of the Shrew" and you have the type. Indeed, Burton played the Bastard at the Old Vic, in 1953. The Bastard has the most lines and, as with Constance, makes "King John" resonate. Final note: "King John" is written completely in verse, the only Shakespearean play besides "Richard II" to be so presented.