'Square Root of Evil' by Richard Harris

Steed is informed by ‘Five’, a colleague of his boss, ‘One-Ten’, that a forger, Riordan, is about to be released from prison, prior to starting work for an organisation that plans to flood the country with forged bank notes. While the forger is taken to a safe-house, Steed takes his place and infiltrates the organisation, gaining the trust of the leader, Hooper. But Hooper's second-in-command, the Cardinal, is suspicious of Steed and has him closely watched. Once he has uncovered their plans, Steed feigns a broken arm suffered while loading a truck, enabling him to reach Doctor Keel and inform him of the plot. After Keel returns with Steed a fight breaks out, and the police are then called in to arrest the villains.

Ian Hendry (Doctor David Keel), Patrick Macnee (John Steed), Heron Carvic (‘5’), Cynthia Bizeray (Secretary), Michael Robbins (Bloom), George Murcell (Hooper), Vic Wise (Warren), Alex Scott (The Cardinal), Delphi Lawrence (Lisa), Ingrid Hafner (Carol Wilson)

Directed by Don Leaver
Produced by Leonard White

TX:
UK: 21st January, 1961

Notes:
*Featuring Doctor David Keel and John Steed

*This episode was broadcast live

*This episode no longer exists