The Avengers.S04E10.Dial a Deadly Number

Steed combats malfeasance in the London financial world, after paged businessmen start dropping dead from heart attacks.
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Teleplay by Roger Marshall
Directed by Don Leaver
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Guest Starring: Clifford Evans, Jan Holden, Anthony Newlands, John Carson, Peter Bowles, Gerald Sim, Michael Trubshawe, Norman Chappell, John Bailey, Edward Cast