The Avengers (1961)

S3E4 The Nutshell

Disco, the Director of Operations at the Nutshell, a secret bunker from World War III, informs Steed and Cathy that a file known as Big Ben, which contains information about their double agents, has been duplicated. Initial investigation leads to a young girl captured on security footage, but she is later discovered dead. As the inquiry unfolds, clues suggest that Steed may be involved, prompting Disco to give the order for his arrest.