This serial replaced a four-part story called The Lost Legion set in a French Foreign Legion outpost which involved a campaign between the alien Skarkel and Khoorians. Sarah was to die at the end of the story, with The Doctor burning her body on funeral pyre. However, Elisabeth Sladen didn't want Sarah to be killed or married off, nor did she want her departure to be the focus of the story.
Usually in Doctor Who (1963), a quarry is used to portray an alien world. This is one of the few occasions where a quarry is an actual quarry.
The last story of Sarah Jane Smith as a regular.
The original script for the story was set in the 1990s at a time when technology and the military are forbidden. Sarah is sent to live in a commune while the Doctor is despatched to a labour camp. There he meets the aged Brigadier - now part of EXIT, the Extraterrestrial Xenological Intelligence Taskforce - and discovers that an anthropologist named Mountford has unearthed a mysterious fossilised hand. The hand takes control of Mountford's mind and forces him to transport it to the Nuton nuclear reactor (previously seen in The Claws Of Axos), which is in the process of being decommissioned. There, the radiation allows the hand to regenerate into its original form: a creature called an Omegan made of teryllium, which has travelled to Earth from inside a black hole. It transpires that there are actually two Omegans at work on Earth, representing different factions of their people. The "hawk" Omegans wish to destroy humanity, while the "dove" Omegans simply want to remove mankind as an interstellar threat. They have accomplished this by slowly devolving men into ape-like Trogs, which manifested itself early on as the backlash again science. Sarah is now undergoing the same transformation. This is undone, however, when the "hawk" Omegan (who crashlanded on Earth, necessitating his reconstitution at Nuton) destroys his "dove" counterpart. He then flees Earth in the other Omegan's spaceship, having configured Nuton to explode and obliterate the planet. At the last second, the Doctor manages to redirect the power of the blast to fuel an experimental rocket called the Icarus. Brigadier Lethbridge Stewart commandeers the Icarus and uses it to pursue the Omegan, ultimately sacrificing himself by ramming the enemy vessel head-on, preventing it from impacting with the Earth. Lieutenant Hawker, who had been a central character in the original draft, was largely replaced with former companion Harry Sullivan. Along with the calcified hand, an Omegan spaceship (referred to as "the Monolith") was now discovered at the start of episode one, and became central to the storyline, serving as the location of the adventure's climax. Bob Baker and Dave Martin also introduced a new supporting character in the form of a Time Lord named Drax. An untrustworthy Gallifreyan mechanic who wants to steal the TARDIS, Drax was conceived as a possible recurring character for Doctor Who.