Terry Nation—creator of the Daleks and the writer of the first season's The Daleks (1963–1964) and second season's The Dalek Invasion of Earth (1964) and The Chase (1965)—to co-write the third season's The Daleks' Master Plan (1965–1966), and it was decided that the extra episode would be used as a cutaway to set up the 12-part story. By April 1965, Donald Tosh was in the process of replacing Spooner as story editor. The episode's production code changed throughout production: "DC" around April and May 1965, "T/A" in July, and later "T Episode 5". The episode was produced by the same team as its predecessor, Galaxy 4; the two serials were the final to be filmed in the show's second production block, but were held back to open its third season in September. Mervyn Pinfield—an experienced BBC figure who acted as the show's associate producer from its origins to January 1965—was originally assigned to direct both serials, but his failing health prevented him from continuing, and Lambert brought on new director Derek Martinus to direct the five episodes. Martinus had recently completed the BBC's internal directors' course, and had no previous experience leading a television production. Having only seen a few episodes of Doctor Who, Martinus reviewed some of the previous stories with Lambert; he found them disappointing, which shocked Lambert, but stated that he wanted to aim for higher standards. Though Lambert was credited for "Mission to the Unknown"—the final story for which she was credited—her role had essentially been replaced by John Wiles. Nation was commissioned to write the episode—then known as "Dalek Cut-Away"—on 25 February 1965; he delivered the script by its due date of 14 May, having also been writing for The Saint at the time. He approached the episode as a technical problem, requiring a small cast and self-contained narrative. The success of the Daleks at this time—particularly due to their imminent American debut in the film Dr. Who and the Daleks (1965)—led Nation to consider the viability of a Dalek-focused spin-off; in writing "Mission to the Unknown", he realised that such a story would need a hero. Inspired by the recent release of the film Goldfinger (1964), he wrote Marc Cory as a "space-age Bond". The episode was originally set on the planet Varga, the home of the Varga plants, but this was renamed to Kemble (later respelled as Kembel) during script revisions for The