Here's the opening mini-biography of King Stephen from Wikipedia:
Stephen, often referred to in history as Stephen of Blois, (c. 1096 – 25 October 1154), was the last Norman King of England. He reigned from 1135 to 1154 and was succeeded by his cousin Henry II, the first of the Angevin or Plantagenet Kings. Stephen was also the Count of Boulogne by marriage.
There were three principal contenders for the succession of Henry I and one 'fancied outsider'. The least popular of these being Empress Matilda, not only because she was a woman, but also because her husband Geoffrey Count Of Anjou was an enemy of the Normans. The other contenders were two men of royal birth, Robert Earl of Gloucester and Stephen himself. The 'outsider' was the elder brother of Stephen, Theobald, Count of Blois. However, Theobald did not want the kingdom, at least not badly enough to contend for it.
Before the death of King Henry I of England in 1135, the majority of the barons of England swore to support Henry's daughter Empress Matilda, (granddaughter of William the Conqueror), and her claim to the throne. However, upon the King's death, Stephen—also a grandchild of The Conqueror—laid claim to the throne, stating that Henry had changed his mind on his deathbed and named Stephen as his heir. Once crowned, Stephen gained the support of the majority of the barons as well as Pope Innocent II and the first few years of his reign were peaceful.
The rest of it can be found here.