“Official Secrets” (*** out of four) was a taut melodrama set in 2003 based on the true story of a British whistleblower (Keira Knightley) for the GCHQ who leaked information to the press about an illegal NSA spy operation that pushed the United Nations security counsel into invading Iraq and how this led to a firestorm of controversy. Absorbing and compelling story never quite cuts loose but takes its low-key grip from Knightley in one of her strongest performances. Matthew Goode, Ralph Fiennes, and Jeremy Northam round out the impressive supporting cast.