For the 16th series of Doctor Who, producer Graham Williams decided to create a linking narrative between the stories, rather like the story arcs used in present day Who (although not as successfully as messrs Davies and Moffat).
The Doctor's intended holiday is put on hold when he is summoned by the mysterious White Guardian, who asks him to locate the six missing segments of the key to time, which if made complete would be a weapon too dangerous for anyone to possess.
The White Guardian imposes a new companion on The Doctor in the (frankly gorgeous) shape of Romanadvortrelundar, or Romana for short played by the glamorous Mary Tamm.
The story, penned by long term Who script writer Robert Holmes is set on the planet Ribos, and centres around a pair of con men trying to sell the icy planet to the tyrant war monger the Graff Vynda-K.
Although the 16th series divides opinion, this story for me is one of the strongest of the 1978 run, and superior to Holmes' other effort that year The Power Of Kroll. Ian Cuthbertson and Nigel Plaskitt as dodgy duo Garron and Unstoffe are marvellous, Paul Seed, who plays the Graff (And would later go on to direct classic BBC drama House Of Cards) is somewhat shouty, but nonetheless entertaining.
The DVD is chock full of extras the stand out for me was 'A matter of time' an hour long documentary looking back on Graham Williams' time in charge of the programme, including interviews with Tom Baker, Mary Tamm, Lalla Ward and John Leeson, and archive interviews with the late Graham Williams and Douglas Adams.
The documentary, written by Nicholas Pegg, gives a very good insight into what the programme making process was like in the 1970's. The producer had to contend with ever diminishing budgets, a difficult to handle lead actor and problems caused by industrial action when the unions were at the peak of their powers. Graham's widow Jacqueline speaks very eloquently about how very tough it was for her husband, and how he cared very deeply for the ongoing success of the programme.
Other extras include a commentary with Tom Baker and Mary Tamm, 'The Ribos File' a making of documentary, continuities and a series 16 trailer.
While the Key To Time series is not perhaps the best in the programme's long history it is still very entertaining, and this DVD boxset is a real "must have" for fans in terms of the extensive extras.