'Double Danger' by Gerald Verner

While treating a patient’s wounds, Doctor Keel discovers that he was a part of a recent diamond heist. Keel gets the man's partner to fetch a prescription from a hospital; but then his assistant Carol notices a cryptic entry on the prescription: ‘Phonus Equus’, which is Latin for ‘call Steed’ - Keel does just that, and informs his partner of the location of the diamonds. But when the man's partner returns to find that the injured man has died, he realises that Keel must have disclosed the location of the diamond's whereabouts, and so kidnaps Carol, threatening to kill her if unless Keel reveals the location to him. Keel is forced to reveal his information, but when the two of them arrive at the hiding place, they find Steed and the police waiting for them.

Ian Hendry (Doctor David Keel), Patrick Macnee (John Steed), Charles Hodgson (Mark Crawford), Robert Mill (Harry Dew), Peter Reynolds (Al Brady), Ronald Pember (Bert Mills), Ingrid Hafner (Carol Wilson), Vanda Hudson (Lola Carrington), Kevin Brennan (Bruton), Gordon Phillot (Bartholomew)

Directed by Roger Jenkins
Produced by Leonard White

TX:
UK: 8th July, 1961

Notes:
*Featuring Doctor David Keel and John Steed

*This episode no longer exists