Officially it is coming because the bureaucratic machine which runs these things has popped its name out of the computer which schedules troop movements on a rotation basis. In reality, the regiment, which also presided over the end of empire in India, is coming because Britain is still good at putting on a military show and few regiments look better on the parade ground than men in kilts.
The Black Watch will provide the backbone of two ceremonies on 30 June, the last day of colonial rule. The first will be the traditional flag- lowering. It will take place at sunset against a backdrop of Victoria Harbour, where Captain Elliot first sailed in to take possession of what a very annoyed Palmerston described as a "barren rock".
The "rock" is now filled to bursting with skyscrapers which crowd round the HMS Tamar military headquarters, where a joint military and civilian ceremony will draw to a close as "The Last Post" is blasted out for the last time. "It will be one of those 'not a dry face in the house occasions'," said Roger Goodwin, the garrison's spokesman.
Joining the Pipes and Drums of the Black Watch will be the Band of the Scots Guards, the Highland Band and the Gurkha Band, all of which will have been flown in because the rundown of the garrison will have reached a stage where bodies are thin on the ground.
As midnight draws near, a much smaller guard of honour will be sent to the extension of the Hong Kong Convention and Exhibition Centre, where they will join an equal number of People's Liberation Army (PLA) troops. Before some 4,000 dignitaries, the British will lower the Union flag and the flag of the colony of Hong Kong, followed by the raising of China's flag and the new flag of the Special Administrative Region of Hong Kong.
Because Britain and China had difficulty agreeing on the handover ceremony , the small- scale military display is the result of many compromises. China initially wanted something on an even smaller scale. Britain had visions of a bigger outdoor event, albeit one which played down any suggestion of British military triumphalism.
The upshot is a slightly awkward indoor event which may well require air blasts to make sure the flags unfurl in an appropriate way.
China is keeping its powder dry for what is expected to be at least one much larger military ceremony next day. Within minutes of the Chinese flags being raised the British will have to be off the premises. The Royal Yacht Britannia will be tied up alongside the Convention Centre, allowing the Prince of Wales, the Governor, Chris Patten, and the Commander of British Forces to make a dignified exit.
They will sail out of the harbour accompanied by the Navy's three locally based patrol vessels (which are likely to be sold to the Philippines), HMS Chatham, a 22 frigate, and the Royal Fleet Auxiliary Sir Percivale. The latter will have been loaded with the remnants of the military's heavy equipment, including a mobile studio for the British Forces Broadcasting Service, which will continue its broadcasts until the last day.
As for HMS Chatham, it will have been pressed into service as a sort of floating office block and command centre, so that the military headquarters can be fully vacated.
The Commander's office will have been transferred, leaving the HMS Tamar complex to the cleaners and backroom personnel whose job it is to ensure that nothing of a sensitive nature is left behind for the PLA.
Chatham and Percivale are being detached from Operation Oceanwave, a full-scale British naval exercise which will be in the area.
There has been much speculation about the timing and location of this exercise but Mr Goodwin insists that it is nothing more than "a helpful coincidence".
As Britannia makes its way to a nearby port, probably in the Philippines, the lower forms of military pond life will be hustled on to chartered aircraft at Kai Tak airport, where they will be given permission to break the midnight flight curfew and fly directly back to Britain. Precise planning for all these events has now moved into high gear, with those responsible being all too well aware of Robert Burns's strictures about "the best laid schemes o' mice an' men ..."