A secret scheme to do something (especially something underhand or illegal)(noun)
WordNet. Princeton University. 2010.
Use "secret plan" in a sentence
"For bin Laden, the 1916 Sykes-Picot Agreement, the secret plan to carve up the disintegrating Ottoman Empire between the French and British, has the same resonance that the 1919 Treaty of Versailles did for Hitler."
"More worrying was the suspicion that they attracted from the authorities, as there were numerous rumours that the Templars had a secret plan to take revenge on the French monarchy and the papacy for the suppression of their Order and the execution of Jacques de Molay."
"In a hilarious, deadpan, self-mocking speech to the Gridiron Club in Washington, he was widely reported to have said he had a secret plan to win the White House: he would claim credit for all the good stuff that had happened in the past eight years and dissociate himself from the bad stuff."