The unusual nature of season sixteen lies in the fact that its six individual stories are linked by a single over-arching plot. This idea had been conceived by Graham Williams when he was first appointed as producer and had originally been considered as the basis for season fifteen.
The three page proposal put together by Williams for this purpose was accepted by Head of Serials Graeme McDonald, but the enforced abandonment of Terrance Dicks's vampire tale The Witch Lords and the shortage of time remaining for advance planning meant that it could not in the end be taken forward for that season. Williams therefore decided to hold it over and use it for season sixteen instead.
The basic premise of Williams' proposal was that there should be a higher power above the Time Lords, and that this should consist of two equal but opposing forces - the White Guardian, standing for 'good' and 'construction', and the Black Guardian, standing for 'evil' and 'destruction' - holding the cosmos in balance. The Doctor would be given by the White Guardian the task of retrieving the six scattered segments of the cube-shaped Key to Time - the source of the Guardians' power - and reassembling them in order to prevent the balance from being destroyed.
Each of the six individual stories making up the twenty-six episode season would revolve around the search for one of the six segments but would be sufficiently self-contained to be enjoyed in its own right. What Williams did not have at the outset, however, was a clear idea of how the over-arching plot of the quest for the six segments should be resolved - that was something that would have to be worked out during the scripting of the season...
The new character devised by Graham Williams as a replacement for Leela (after he had unsuccessfully attempted to persuade Elisabeth Sladen to return as Sarah Jane Smith) was a female Time Lord named Romanadvoratrelundar (or Romana for short) - the first companion of the Doctor's own race since his grand-daughter Susan in the series' first ten stories. He envisaged her as an 'ice maiden' in the Grace Kelly mould, whose academic prowess would be balanced by naivety and a lack of practical experience. The actress chosen for the part, apparently out of some three thousand applicants, was Mary Tamm, previously best known for her role in the 1974 Columbia film production The Odessa File.
K9 was also kept on as a companion - a relatively late decision - after Williams received assurances from the Visual Effects Department that the prop's internal workings could be redesigned to make it quieter and more controllable. This refit was carried out in collaboration with a firm called Slough Radio Control, who allowed employee Nigel Brackley to be seconded to Doctor Who on a semi-permanent basis to look after the mechanical dog. This season would also see Tom Baker matching his predecessor Jon Pertwee's five year stint on the series, well on his way to becoming the longest-serving of all the Doctors.
Anthony Read's first full season as Doctor Who's script editor was also his last; he bowed out on The Armageddon Factor and handed over the reins to his successor, Douglas Adams.
Production of season sixteen had been relatively trouble-free by comparison with that of the previous one, but Graham Williams nevertheless considered that the use of a linking theme had imposed too many strictures to be repeated on a regular basis. Season seventeen would therefore see a return to the more familiar format of a succession of unconnected stories, the transmission order of which could if necessary be changed during the course of production. This was perhaps just as well, as production of that season would prove to be anything but trouble-free...