The Hatfield House Mystery
What exactly happened at Hatfield House, the ancestral home of the Salisbury family, when Viscount Cranborne, a former prominent opponent of the Good Friday Agreement and a senior Tory, hosted a meeting between the Tory front bench, the Ulster Unionists and Peter Robinson's DUP?
That is a question that is causing tremors in Belfast at the moment; it also has consequences for the UK as a whole. When asked about it this morning, David Cameron did not deny that the issue of what unionists might do in a hung Parliament could have been discussed.
Instead, he said that his focus was on ensuring that devolution to Stormont continued successfully and on resolving differences between the two unionist parties.
Some have suspected that the Tories, who have gone into alliance with the Ulster Unionists for the forthcoming general election, were eyeing a deal whereby different unionist parties would stand down for each other in different Northern Irish constituencies.
The Tories deny this, insisting that if the Conservative name is used, they will fight every seat in Northern Ireland. Watch this space. This matters, even if you don't live in Northern Ireland.