A major underlying theme that has slowly been building steam is the comparison between the colony on Frontios to that of an Orwellian, 1984-type society, complete with buzz phrases like "deaths unaccountable" and "retrogrades" and massive portraits of the late Captain Revere like the one found in the hall of justice.
To focus on this subplot would have made Frontios a very interesting viewing experience, but instead, Episode Three sees the story of the Tractators take over. The Tractators are somewhat ridiculous looking creatures, especially their leader, the Gravis. It's one thing that the mobility of these creatures is severely limited and their shells are quite clearly fiberglass, but to give the Gravis a nose and ears to try and set him apart from his non-speaking brethren was a very poor decision. The Tractators are the 1980s equivalent to the Zarbi.
Still, this episode contains something of a rarity - scenes featuring just The Doctor and Tegan. Despite being on the show for three years, Tegan rarely spends any one-on-one time with The Doctor, either because she's separated from him during the course of the story, or because she was never the only companion in the TARDIS during her entire time on Doctor Who. It's a shame, really, because Peter Davison and Janet Fielding make a good team in this episode, and it's easy to see why they're continued their amusing relationship in DVD commentaries today.