On January 20, 1843, Edward Drummond, private secretary to then-English Prime Minister Sir Robert Peel, was shot in the back as he was approaching the Prime Minister's official residence at 10 Downing Street. Drummond was able to make his way to his family's nearby bank where he collapsed. The shooter, Daniel M'Naghten made no attempt to flee the scene and was quickly arrested. It soon became apparent that Drummond had been shot by mistake as M'Naghten had believed him to be the Prime Minister. After Drummond's death five days later, Daniel M'Naghten was formally charged with murder.
The trial at the Old Bailey was held on March 3 of that year and the courtroom was packed as the charges were read. Despite the Crown's attempts to portray him as mentally competent, evidence presented by the defense was able to establish that M'Naghten had believed Sir Robert Peel was part of a vast conspiracy against him (along with the British Tory government and Jesuit priests). Testimony was heard from from various psychiatrists and the presiding judge concluded that enough evidence had been heard to bring in a verdict of "Not Guilty by Reason of Insanity". Given the political nature of the crime, there was considerable public opposition to the acquittal. Queen Victoria in particular was greatly upset (she had survived an assassination attempt by a mentally ill man in 1840) and requested that the case by reviewed by the House of Lords.
It was as a result of the House of Lords proceedings into the legal basis of insanity that the M'Naghten rule of insanity was formally enshrined into law. Based on the rule, a defense of insanity could only be permitted if it could be proven that the offender was sufficiently impaired as to not know the nature and quality of the act that he or she was doing or did not know that the act was wrong. The M'Naghten rule would form the standard for insanity pleas in most English-speaking jurisdictions until well into the late 20th century. While many American states still use a variation of the M'Naghten rule, it has been largely superseded in the British court system.
As for Daniel M'Naghten? He was forcibly institutionalized for the rest of his life under the Criminal Lunatics Act of 1800. He would spend twenty years at Bethlem Royal Hospital before being transferred to Broadmoor where he would die in 1865. His own reaction to becoming part of legal history does not seem to be recorded.