The Queen -v- Patrick O'Donnell reveals on screen for the first time the true story behind one of the most compelling murder plots in Irish history, a thrilling tale of violence, courtroom drama, romance and political intrigue. Containing evidence from British Home Office files kept secret for 100 years, it reveals shocking new information about the fate of the quiet-spoken Donegal man who killed the infamous informer James Carey on board a ship off the coast of South Africa in 1883. What possessed the illiterate labourer from the Donegal Gaeltacht to shoot the leader of The Invincibles? Who was the mystery woman posing as his wife? And why did both the US President and the French luminary Victor Hugo plead that Patrick O'Donnell not be hanged for his crime?

In darkest rural Ireland, ex-boxer Douglas 'Arm' Armstrong has become the feared enforcer for the drug-dealing Devers family, whilst also trying to be a good father to his autistic five-year-old son, Jack. Torn between these two families, Arm's loyalties are truly tested when he is asked to kill for the first time.