In June 1405, King Henry IV stopped at a small Yorkshire manor house to shelter from a storm. That night he awoke screaming that traitors were burning his skin. His instinctive belief that he was being poisoned was understandable: he had already survived at least eight plots to dethrone or kill him in the first six years of his reign.

Henry IV had not always been so unpopular. In his youth he had been a great chivalric champion and crusader. The son of John of Gaunt, he was courteous, confident, well-educated, generous, devoted to his family, musical and spiritually fervent. In 1399, at the age of thirty-two, he was enthusiastically greeted as the saviour of the realm when he ousted from power the insecure and tyrannical King Richard II.

But therein lay Henry's weakness. He had to contend with men who supported him only as long as they could control him; when they failed, they plotted to kill him. Welsh, French and Scottish adversaries also tried to take advantage of his questionable right to the crown. Such overwhelming threats transformed him from a hero into a duplicitous murderer: a king prepared to go to any lengths to save his family and his throne.

That legacy of unrest has defined Henry's subsequent reputation. Henry's notoriety in the sixteenth century was such that merely to write about him was to risk imprisonment in the Tower. Shakespeare was forced to downplay his achievements, and instead to present his adversary Richard II as the wronged man. But what Henry actually provoked was a social revolution as much as a political one. Against all the odds, he took a poorly ruled nation, established a new Lancastrian dynasty, and introduced the principle that a king must act in accordance with parliament. He might not have been the most glorious king England ever had, but he was one of the bravest, and certainly the greatest survivor of them all.

Dr Ian Mortimer is theSunday Timesbestselling author ofTheTime Travellers Guide to Medieval EnglandandThe Time Travellers Guide to Elizabethan England, as well as four critically acclaimed medieval biographies, and numerous scholarly articles on subjects ranging in date from the twelfth to the twentieth centuries.

He was elected a Fellow of the Royal Historical Society in 1998. His work on the social history of medicine won the Alexander Prize (2004) and was published by the Royal Historical Society in 2009. He lives with his wife and three children in Moretonhampstead, on the edge of Dartmoor.