Thomas Becket came from humble beginnings. Sexual interest by an aristocrat in his sister led to his being educated in the knowledge and culture of the higher classes, and a connection with the then-Archbishop of Canterbury led to diplomatic work for the Church and, ultimately, an introduction to King Henry II and a post as Lord Chancellor.
Becket set about his new post with gusto, becoming an amusing companion and bon vivant as well as an efficient and devoted administrator. He and the King became good friends, with Henry even sending his eldest son to live in Becket's household.
A turning point came when the old Archbishop of Canterbury died and Becket was appointed in his place. Confident that his friend would look out for his interests, Henry planned to diminish the independence of the Church in England. Becket, however, underwent a complete change in external character, becoming a stern, virtuous ascetic, devoted to the interests of the Church above all things. A series of legal conflicts led to personal conflicts and the rift in the famous friendship began.
Becket was stubborn and a bit fanatical, and even annoyed the Pope himself with his opinionated pursuit of what he deemed to be the Church's best interest. It came to a head with the Constitutions of Clarendon, a set of legislative procedures that would restrict ecclesiastical privilege in England. The King demanded he sign the document; Becket refused.
Over the next seven years the two great men struggled and feuded. Becket spent much of this time outside of England, being protected by the King's enemies and trying to persuade the Pope to excommunicate the whole country. It all came to a head in 1170 when the king uttered the fateful question, "Who will rid me of this troublesome priest?" — or words to that effect. There are many different versions but he definitely said something. Most historians agree that the King's outburst was not spoken with intent, but four knights interpreted it as a royal command and set about assassinating Becket. They ambushed him in Canterbury Cathedral. A bystander, who was wounded in the attack, wrote:
"The wicked knight leapt suddenly upon him, cutting off the top of the crown which the unction of sacred chrism had dedicated to God. Next he received a second blow on the head, but still he stood firm and immovable. At the third blow he fell on his knees and elbows, offering himself a living sacrifice, and saying in a low voice, 'For the name of Jesus and the protection of the Church, I am ready to embrace death.' But the third knight inflicted a terrible wound as he lay prostrate. By this stroke, the crown of his head was separated from the head in such a way that the blood white with the brain, and the brain no less red from the blood, dyed the floor of the cathedral. The same clerk who had entered with the knights placed his foot on the neck of the holy priest and precious martyr, and, horrible to relate, scattered the brains and blood about the pavements, crying to the others, 'Let us away, knights; this fellow will arise no more.'"
The crime haunted King Henry for the remaining 20 years of his life and reign. The public, both in England and outside of England, held him accountable for the murder. When it was discovered that Becket wore a hairshirt (a rough, uncomfortable garment made of animal hair) under his clerical finery, the public went mad and declared him a martyr, clamouring for his canonisation, which took place just three years after his death.
Henry, desperate to regain some kind of public standing, humbled himself with a public penance at Becket's tomb in July of 1174. But he had lost ground that he never fully regained. Most significantly, his eldest son Henry, who had grown up in Becket's household, hated him for it. Henry died in 1189, nearly 20 years after Becket, weak, ill, and estranged from all but one bastard son.