The Avengers.S06E08.All Done with Mirrors

Secrets are leaking from a defence research establishment, thanks to a new invention: an eavesdropping device, which can use any shiny surface to reflect and amplify sound waves. But with Steed unavailable, Tara King must investigate with only an inexperienced new agent for support.
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Teleplay by Leigh Vance
Directed by Ray Austin
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Guest Starring: Dinsdale Landen, Peter Copley, Edwin Richfield, Michael Trubshawe, Nora Nicholson, Tenniel Evans, Michael Nightingale, Robert Sidaway, Edina Ronay (uncredited), David Grey, Graham Ashley, Peter Thomas