A paramilitary organisation operating on the fringes of the SNP, the Scottish Liberation Army, stages a rising, seizing Fort William.
The BBC adapted it for television in 1973, but has not reaired it since its original transmission due to an upheld complaint that it could mislead viewers about the political views of the SNP.
MI5 agent Graham Hart and City of Glasgow Police Detective Superintendent Rennie infiltrate a demolition expert named MacNair into the SLA.
Mackie's girlfriend Susan ("Sukey") Dunmayne, daughter of a laird, briefs an SLA section in Stirling, consisting mainly of students and commanded by effete lecturer Donald Levi, to act against any deal between Henderson, the moderate leader of the SNP, and the sitting Conservative Prime Minister Patrick Harvey.
Brodie discovers a paper trail left by Hart's clumsy attempt to establish a cover story for MacNair's past.
Although semi-retired, Thorganby still has political influence, and when he reads out Mackie's letter in front of the delegates, he thwarts any possibility of the Conservatives ratifying the deal worked out at Hexham.
Donald Levi has tried to emigrate to Cuba via France, but has been forced to return to Scotland with a letter from Bucholz for Sukey.
Following Thorganby's appointment and the promise of a hard line being taken with pro-independence agitation, Mackie has decided that the time is right for the SLA to launch an armed insurrection.
He forces Thorganby to walk to English lines in his pyjamas in full view of the press and TV cameras, to ensure maximum humiliation and publicity.
However, Brodie, who had been placed under arrest by the SLA for insubordination, has deserted and brought with him Bucholz's letter, which MacNair had dropped.
He then reads selected parts of the letter on television, stating that the avowed aim of Bucholz and Mackie was to establish a socialist dictatorship in Scotland.
The broadcast also induces many recent recruits to desert the SLA, taking advantage of an amnesty announced by Harvey.