Short answers, pulled from the story.
The Conspiration des poignards, also called the Complot de l'Opera, was an alleged plot to assassinate Napoleon Bonaparte as he left the Paris opera house on the 10th of October 1800. The police of Joseph Fouché claimed to have intercepted four armed men waiting outside after a performance of Les Horaces.
The principal figures named were Giuseppe Ceracchi, a Roman sculptor and co-founder of the Roman Republic in 1798; François Topino-Lebrun, a painter and former student of Jacques-Louis David; Dominique Demerville, a former clerk of the Committee of Public Safety; and Joseph Antoine Aréna, whose brother had previously tried to stab Napoleon during the coup of 18 Brumaire.
Modern historians Jean Tulard and Thierry Lentz have concluded it was a manipulation by the police, made possible by an agent provocateur named Harel who had infiltrated the group. This interpretation was questioned even at the time of the events.
Four conspirators were condemned to death on the 9th of January 1801 by the criminal court of the Seine, after three days of debate. Their appeal was rejected and the sentence was carried out on the 30th of January 1801.
Fouché, as head of the police, was credited with foiling the plot. In his later Mémoires he described deploying Harel as an informer to expose the conspirators, though historians have since characterised Harel as an agent provocateur who helped manufacture the plot the police then claimed to stop.
The plot was publicly labelled a Jacobin conspiracy, linking the accused to radical revolutionary politics and helping justify the early Napoleonic state's security apparatus. The defendants were tried before the criminal court of the Seine after the separate royalist Plot of the Rue Saint-Nicaise, framing the daggers case as part of a wider threat to the regime.