In some context they are synonyms to each other. Conspiracy= a secret plan by a group to do something unlawful or harmful: Ex- a conspiracy to destroy the government [mass noun] the action of plotting or conspiring: they were cleared of conspiracy to pervert the course of justice. Scheme :a clever and often dishonest plan to do or get something Ex-a scheme to cheat people out of their money. plot: a secret plan to do something that is usually illegal or harmful . Ex. plots against the government .