secret plan is a noun meaning "a secret scheme to do something (especially something underhand or illegal)". It carries one recorded sense.
a secret scheme to do something (especially something underhand or illegal)
“they concocted a plot to discredit the governor”
“I saw through his little game from the start”
Definitions, synonyms and semantic relations for secret plan come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.