Gregory’s Restoration-era spy, Thomas Chaloner, combines detection with espionage in Murder on High Holborn.

Where did the idea for the plot come from?

It actually came from three curious but true stories. John Scott was a fascinating character, who stumbled from one scam to another, and who ended his days not in a prison where he belonged but as a high-ranking official in the Caribbean. The hapless ship London had just finished a refit in the royal dockyard, and was sailing up the Thames to be feted by the city after which she was named, when she exploded, with the loss of more than 300 lives. No one really knows what happened, but as she was a large warship, her loss was a serious blow to the navy, as war had just been declared on the United Provinces of the Netherlands. Finally, the Fifth Monarchists, a dangerous sect, believed that the execution of Charles I meant that King Jesus was poised to come and establish a heavenly kingdom in Whitehall, and so were vexed when Charles II was crowned, and decided to overthrow the king and his government. The movement attracted some very eccentric and unpleasant fanatics—the kind of characters that are a lot of fun to write about.

How would you describe Chaloner?

Chaloner has lived through the civil wars and the reign of Oliver Cromwell, and is cynical and world-weary. He served as an intelligencer for Cromwell’s regime, and is acutely aware that ex-parliamentarians are persona non grata in Restoration London. He is a competent warrior, and an astute observer of human nature. He is taciturn and withdrawn in his personal friendships, however, and his relationships with women tend to end in disaster. He is skilled at surveillance, and thinks nothing of breaking into people’s houses to find what he wants.

What was Chaloner’s time like?

The 1660s came after two decades of civil war and political uncertainty. Charles II was on the throne, but half the population didn’t want him there—especially once he showed himself to be debauched, hedonistic, and an indifferent ruler, and at the beginning of his reign in particular, his hold on his throne was tenuous. Rebellions bubbled all over the country, and no one knew whether one might succeed and plunge the nation back into civil war.

Which guidelines do you follow when using historical figures?

Most of the characters in my stories are based on real people. In the Chaloner books, there’s more information available than in my other series, set in 14th-century Cambridge, so it is easier to gain some notion of their personalities. However, I’m glad none of them are alive to sue me, as I have taken considerable liberties with most of them.