The commonly held belief that the king or queen is a mere ornament of the constitution could not be further from reality. The turbulent past decade has tested the British constitution like never before, and Brexit, the illegal proroguing of Parliament and the near death of Boris Johnson while in office have demonstrated the often misunderstood but crucial role the sovereign plays in ensuring stability in our political system. The king or queen holds a unique set of powers that enable them to act as the defender of our constitution, a duty they perform not through the active use of power but by constitutional authority. The mere threat of action by the sovereign is often all that is needed to get the political machine back on track and avoid crisis. Taking us on a fascinating journey through centuries of British political history, focusing on the monarch's powers to appoint and dismiss a Prime Minister, grant a dissolution of Parliament and refuse assent to legislation, On His Majesty's Service examines how past monarchs have avoided constitutional crisis and reveals the hidden hand of the monarchy in the smooth running of the state, shattering the accepted political truism that the king reigns but does not rule. These are the king's prerogative powers, and this is their story.