Still, the future lies this way.
Season Eighteen of Doctor Who can be compared to renovating an inhabited house, as producer John Nathan-Turner, aided by script editor Christopher H. Bidmead, measures the windows, selects new carpeting, and pulls out the plumbing while the family already living there tries to get on with daily life. It is, charitably, an uneven season, with constant change from episode to episode, not all of it successful. But for the season finale—and Tom Baker’s final story—Bidmead delivers a striking tale, at once a meditative mood piece and a cracking bit of tense action: “Logopolis” (Story Production Code 5V) delivers on the promise of positive change in the series while providing Baker and the Fourth Doctor with a satisfying conclusion to over six years of adventures through time and space.
The story starts in quotidian fashion, with the Doctor wanting to finally fix the wonky Chameleon Circuit on the TARDIS, allowing it to change shape into something other than a police box. He calls up the never-before-seen Chameleon Circuit panel, which pops out of the central console, to demonstrate what the TARDIS would look like as a pyramid. There’s a real sense of Bidmead wanting to add to the lore of the TARDIS, particularly with his introduction of the Cloister Bell, a staple plot device in years to come, which sounds its warning peals as the Doctor and Adric are talking in an old part of the ship overgrown with vines.
In order to repair the Chameleon Circuit, the Doctor first needs exactingly precise dimensions of the object the TARDIS is stuck externally representing, which necessitates a trip to Earth. Bidmead and director Peter Grimwade, a long-time Doctor Who crew member, go out on location to place a police box (or two) near a motorway leading to “London Airport,” a road along which Australian flight attendant Tegan Jovanka (Janet Fielding) and her Aunt Vanessa (Dolore Whiteman) are driving before a flat tire stops them. It’s no ordinary police box that Tegan enters looking for help, though, as another TARDIS has already materialized around the box that was there.
Rather than jump right into action and excitement, Bidmead slowly unspools the tension, spending the majority of the first of four episodes ruminating on the nature of the TARDIS, of the complications of dimensionality and recursive loops. When the Doctor’s TARDIS arrives at the very same police box, it lands “around” the other TARDIS, which itself landed around the real police box. After several minutes spent watching the Doctor and Adric measuring the police box and discussing the “block transfer computations” needed to reprogram the Chameleon Circuit—a type of mathematics so advanced it can only be done by the living computers of Logopolis—they venture inside the police box and find another TARDIS with a police box inside it, and on and on, a paradox caused by one TARDIS being inside another. The Doctor fears they are trapped in an infinite regression, but finally they pop out back on the motorway, where three police officers have some questions about an abandoned car with two dolls in the front seat…