The story begins in 1960s Britain, a time when Preston says the leaders of the main political parties were "dull plain-looking men without any kind of noticeable charisma".
Jeremy Thorpe was different.
"He was young, he was good-looking, he was charismatic, he had a very beguiling air of mischief about him which was almost an unknown thing at the time," Preston says.
But Thorpe was also a risk taker. While a sitting MP, he had a series of affairs with men — at a time when homosexual acts were still a crime and could lead to a jail sentence.
In the early 1960s he met an 18-year-old stablehand named Norman Josiffe.
"They talked for a bit and Thorpe said, 'If you're ever in any trouble, come and see me at the House of Commons', and gave him his card.
"It so happened that Josiffe had a tremendous falling out with his employer shortly afterwards, he then had a nervous breakdown, his life basically imploded.
"So he went to see Thorpe. To some degree to throw himself on Thorpe's mercy."
Thorpe and Josiffe had an affair, but when the relationship broke off things spiralled out of control.
Josiffe, who was by now known as Norman Scott, struggled to find work and battled depression.
He blamed many of his hardships on Thorpe and told police about their relationship, handing over copies of personal letters as evidence.
Thorpe decided it was time to silence his former lover.
By now he was leader of the Liberal Party and had ambitions of being the next prime minister of Britain — and he could not let those ambitions be destroyed by his past.
"And he siphoned 10,000 pounds out of the Liberal Party funds to pay this unbelievably inept hitman to try and kill Scott.
"However the hitman only succeeded in killing Scott's dog, Rinka.
"The hitman it turned out had a dog phobia.
"He was called Chicken Brain, with some justification, by his friends.
"He shot the dog, then the gun jammed, and he drove off into the night."
The police turned up to the scene and Scott once more "poured out his story", blaming his misfortune on Thorpe's mistreatment of him.