In 1679, Samuel Pepys was at the height of his powers. A celebrated intellectual and member of parliament, he had also until recently been one of the most senior civil servants in the Admiralty. He was admired by King Charles II and close to Charles' brother and successor to the throne, James, Duke of York.

But on May 22 that year his world came crashing around him. He was forced to embark on a legal marathon that started with incarceration in the Tower of London and would soon threaten not just his freedom, but his life. Pepys was thrown into the centre of a political storm that swept through England and would claim the lives of many innocent Catholics.

James and Ben Long tell the fascinating story of a two-year period, when Pepys' life and the future of the British throne hung in the balance.

What was at stake was immense, because his diaries, which are among the most comprehensive accounts of England during the Restoration, lay hidden in his library. Pepys was facing a trial for treason. Had he been found guilty, he would have been hung, drawn and quartered, his lands forfeited to the crown, and his personal papers probably destroyed - the diaries lost forever.

At the centre of this legal struggle were two men: a bitter ex-Jesuit turned anti-Catholic witch-hunter Titus Oates, and a serial liar, thief and coward John Scott, who had given himself the rank of colonel.

Both were major opportunists. When Charles became king in 1660, he was welcomed by his subjects. But rumours grew that, although he publicly adhered to the Protestant faith, he was secretly a Catholic.

Whatever his private beliefs, Charles would never put the monarchy at risk. However, Oates took advantage of the fears of ordinary people and invented what became known as the Popish Plot.

He first suggested that James would impose Catholic rule on succeeding Charles. As he grew bolder - and without a shred of evidence - Oates then claimed there was a plot to kill Charles.

The odds were stacked in his favour, because once charged with treason, it was up to a defendant to prove his innocence. He was not allowed legal representation and only two witnesses were needed to condemn him.

In June 1679, five Catholics were executed when Oates accused them of seeking to kill Charles with silver bullets.

Whereas Oates was on a crusade against an entire religion, Scott's motivation was more personal. He had appeared in court before Pepys, who was a justice of the peace. Falsely accused of murder, he was forced to flee England briefly and blamed Pepys for his misfortunes.

Scott had the support of senior Whig politicians who realised that bringing down those close to James weakened his position and his right of succession.

As more Catholics trooped to the gallows and the London mob bayed for blood, Pepys, even though he was a Protestant, realised that his only salvation lay in discrediting Scott.

What emerges is an absorbing detective story, as Pepys seeks to learn more about his accuser. As the Longs point out, Scott, growing up in the American colonies, had good prospects. He was even tipped to become governor of Long Island, but his inveterate lying tripped him up.

Scott claimed to be heir to an estate and title in England and sold tracts of land in America that he didn't own. One man who'd been duped by him described him as 'a godless, profane man, a false and traitorous suborner'.

Indeed, Scott was guilty of the treason of which he accused Pepys: trying to sell to the French detailed information about the British navy. When his duplicity was exposed, he fled England again and the case against Pepys was dismissed.

The Longs' account isn't without flaws. There are many players in this saga and the authors seem to think we need to know about everyone involved. The first few chapters are tedious, with the Longs failing to selectively set the scene for the legal tussle that would follow.

But it's worth persevering. They paint a vivid picture of a world of intrigue and scandal at the heart of the English Restoration.