Roots: "Cleanliness is not next to godliness" comes via Francis Bacon or John Wesley. The Doctor quotes Aristotle on possibility and paraphrases Hamlet ("Something rotten in the state of Chobel").
Dialogue Triumphs: Tegan's threat - "Do I have to start coughing on these two?"
"Take us to your cleaner"
Dialogue Disasters: As noted by the actors in the extras, some of the lines are deliberate (and unnatural) tongue-twisters: "The Takers have taken Tegan", "Another martyr for Mertil's cause", "It's completely depleted"
Continuity: The planet Cherdor, on the eastern edge of Heldebrin has at least one moon, but no strategic or mineral importance.
Around a hundred years ago while in Dravidian space, forty-four patients aboard an Earth Empire medical ship developed Richter's Syndrome. The on-board battlespace automedics had no protocol for the eventuality of Richter's Syndrome, and so euthanasia was elected. The automedics killed all but two medical staff who were themselves divided on what to do with the who. The medico against invoking standard protocol - active combat regulation 205 (euthanasia) whipped up rebellion among the patients and in the conflict the ship's quantum flux generator was hit by percussive weapon fire, exploding and sending layers of warp energy radiating outwards, transforming the humans about it into intra-dimensional 'contaminants', the 'Shades' of the modern culture, locked in a permanent state of agony and suffering, but empowered through an innate telepathy and the ability to animate fallen Takers, the transformed automedics (the Doctor thinks they exist or operate on a different frequency.) In the ensuing crash and all communication with their central register brought down.
The Takers are still on an active war footing and work under the regulations last given to them by their human masters to the letter - regulation 201 allows them to aggressively neutralise any threat to their patients, but their work remains secret since the first leader of the survivors banished them and expunged any memories of the warp catastrophe to prevent further psychological trauma. Having secured a sterile and idyllic setting to tend their patients the automedics use their teleporting abilities to rescue any injured to fatally-wounded survivor and return them to their quarantine built from wreckage of the original ship. Their red-craniums are actually lights to allow identification in low visibility areas.
Tegan gets a headache due to her proximity to the Shades. At the end of this story she suggests that the Mara is still within her (see: Links) The Doctor has set the TARDIS to "sniff out" any nearby instances of Richter's in order that Nyssa can test her cure.
Future History: During the 27th century the Earth Empire is engaged in conflicts across Dravidian space
Links: The Brain of Morbius (Dravidians), Gridlock (the green crescent is a universal symbol for medical care). Tegan refers to and speaks with the voice of, the Mara (Kinda, Snakedance), this story leading into the opening of Cradle of the Snake.
Location: The planet Cherdor, in the 28th century
The Bottom Line: "Find. Heal. Take flesh, make fresh..."
Traditional, but given the predecessor it has to live up to, surprisingly inventive. There's a sense that perhaps the TARDIS team is a little too large (as it was in the TV series of course), but newly-returned Tegan is treated well, with some fun scenes, and for that at least this is worth a re-listen. Plus, the ending is a kicker.