On a frigid January day on London's Whitehall in 1843, a Scottish woodturner named Daniel M'Naghten guns down Edward Drummond, believing him to be British Prime Minister Robert Peel. M'Naghten, who sympathizes with the Chartist cause in Great Britain, claims he intended to murder the Prime Minister-a Tory-because he blames Peel for persecution by the Tories in his home city of Glasgow. Queen Victoria, incensed at the most recent attempt on a high government official's life, demands that M'Naghten be hanged. M'Naghten, however, demonstrates every accepted sign of insanity, which would save him a visit to Tyburn Tree. Queen's Counsel Alexander Cockburn is hired to defend M'Naghten, and he recruits legendary thief-taker Vicar Brekonridge to travel to Glasgow to investigate M'Naghten's claims, with the goal of supporting an insanity plea. He sends young law clerk Simon Daughtrey to Glasgow with Brekonridge, and together they uncover contradictory evidence suggesting that M'Naghten's motivations in the murder of Edward Drummond might be considerably more sinister than mental illness. With M'Naghten's trial days away, Brekonridge and Daughtrey race to find the truth behind the assassination of Edward Drummond.