The Avengers (1961)

Season 6: Episode 25

Whilst on a top secret security assignment, Tara comes under suspicion of being a double-agent. She must cast similar suspicion on Steed if she is to prove that she has been framed. An enemy agent, Gregor Zaroff, hopes to put the Government's new anti-missile defence system, code named 'Field Marshall', out of action by convincing Steed and Mother that Tara has betrayed the system's secrets. (Source: en.wikipedia.de)