A plot to murder the Tory Prime Minister, Lord Liverpool, and the whole cabinet, as they had dinner at a house on Grosvenor Square. The dinner had been announced in a newspaper but this was probably common practice with many government and social activities. The conspirators were the Spencean Philanthropists, named after Thomas Spence and led by Arthur Thistlewood. The objective was a revolution, like that in France. Infiltrated by a government agent the gang was apprehended at its base in Cato Street. More details at the picture source website and at A London Inheritance.
The Grosvenor Square house was number 39 (renumbered as 44 in 1888), the home of Lord Harrowby. These dinners must have been regular events because five years earlier, the cabinet were here having just such a dinner, and were interrupted to be told that Napoleon had been defeated at the Battle of Waterloo.
'Celebrated Trials of all Countries, and Remarkable Cases of Criminal Jurisprudence. Selected by a member of the Philadelphia bar', 1835, by J. J. Smith quotes Lord Harrowby: "I reside in Grosvenor-square. I am president of the council, and a member of the cabinet. On Wednesday, the 23rd of February, I was to have had at my house a cabinet dinner, and cards of invitation had been issued to the following personages:.. At a cabinet dinner, none but the members of the cabinet were invited. On the Tuesday before the intended dinner, I was riding in the Park about two o-clock, preparatory to my attending a council. I had no servant with me. A person addressed me near Grosvenor-gate, and said he had a letter addressed to lord Castlereagh. The letter now in court is that letter ... The cabinet dinner did not take place as intended on the Wednesday, but the preparations for it were carried on in my house just as if it was to take place; nor did I countermand them until eight o-clock on that evening, when I wrote a note to my head servant, from lord Liverpool's."