If You're A Fan of the Brother Cadfael Mystery Books...

...you might wish to check out this long YouTube video on the background history of England at the time, called "The Anarchy." It's a fun telling with some good animation of battles and skirmishes. To summarize it, Henry I, son of William the Conqueror, was predeceased by his legitimate son (Henry had numerous bastards, but they couldn't inherit). So Henry secured promises from his vassals to acknowledge his daughter Matilda as queen of England after he died. One of them, Stephen of Blois, usurped the throne after Henry's death, and a civil war ensued that lasted over ten years, with the issue only being settled when Stephen agreed to adopt Henry's grandson Henry Plantagenet as his heir (Stephen's own son Eustace had predeceased him). This was acceptable to all parties, and when Stephen died, Henry Plantagenet became Henry II.