Episode 29, "The Sell-Out," was the third and last to feature Jon Rollason's Dr. Martin King (but the first to be broadcast), as well as the last to feature a male partner for Steed (Patrick Macnee, for the third straight time, receives sole billing in the opening credits). While Ian Hendry's Dr. David Keel was often seen in his surgery with Ingrid Hafner's nurse Carol Wilson, this would prove to be the only time that Steed would visit Dr. King, offering a final look at Gillian Muir's nurse, Judy, barely noticeable in her back-to-back appearances. This time, Dr. King aids Steed in ensuring the safety of a high ranking French official (Carleton Hobbs) attending a secret peace conference in London for the UN. Michael Mellinger (previously seen in "The Far-Distant Dead") plays the barber shop informant who trails Steed, and Frank Gatliff ("One for the Mortuary," "A Chorus of Frogs," and "Love All") heads up the security team. Arthur Hewlett makes his only series appearance as One Twelve, Steed's superior, who scoffs at the notion that there might be a traitorous informer sabotaging their security efforts. Often in these early efforts, and prior to the final season introduction of Patrick Newell's popular Mother, Steed would take his orders from an on-screen superior, the most frequent being One Ten, played in ten episodes by Douglas Muir ("Diamond Cut Diamond," "The Springers," "Death on the Slipway," "The Tunnel of Fear," and "The Deadly Air," all from season one, with "Death Dispatch," "Warlock," "Mr Teddy Bear," "The Removal Men," and "Immortal Clay" from season two). One Six (Michael Gover) appeared twice ("Man in the Mirror" and "A Chorus of Frogs"), Charles (Paul Whitsun-Jones) appeared twice ("Man with Two Shadows" and "The Wringer"), while One Twelve, One Seven (Frederick Farley in "School for Traitors"), One Fifteen (Eric Dodson in the lost first season enrty "Dragonsfield"), and Quilpie (Ronald Radd in "The Outside-In Man") made just one appearance each. One Twelve remains a deliberately shady character, whom Steed admits to never having worked with before (and never would again). There are too few red herrings, and while it may be a shame that the partnership between Steed and Dr. King was so necessarily brief, subsequent casting proved that the show's popularity would endure beyond its 8 years on the air.