Season 16 is a bit of a departure from DW at the time in that this series, for the first time, has a continuing story arc throughout all six serials. This is certainly common in modern DW, but at the time was quite an experiment.
The White Guardian enlists the Doctor to search for and collect the six segments to the Key to Time and gives him an assistant and a locator device, the Tracer, to do so. Enter the lovely Mary Tamm as Time Lady Romanadvoratrelundar, aka Romana, aka Fred! The Doctor also has a new K-9, since he left the Mark I model with Leela on Gallifrey.
The story itself is a rather tight, straightforward adventure from Robert Holmes filled with many interesting characters played to perfection by an outstanding guest cast. I've always found it to be quite fun and one of my favorites from this particular series. The sets and costumes are quite wonderful, too.
Much love to Mary Tamm's portrayal of Romana. Although somewhat haughty, she is the first companion since Liz Shaw to be more of the Doctor's equal in terms of strength and scientific ability. Oh...and she's stunningly gorgeous, too. I loved her in all six of these stories and like Liz, she only lasted the ONE year.
