blow someone's cover

Iskander's role as a G.A.M. operative was secret—until I unwittingly blew his cover that day.

This spy film purports to be inspired by the true story of Kim Philby (1912-1988), a British intelligence officer and Soviet spy during the 1940s and '50s who gained international notoriety when he blew his cover and defected to the USSR in 1963.