Roots: The works of H P Lovecraft (necromantic tomes, the Ancient Ones, the phrase "cyclopean architecture" appears in The Call of Cthulhu, particularly At The Mountains of Madness, with its polar setting and penguins, the description "cacodaemoniacal" derives from his short story The Hound.) Raiders of the Lost Ark ('Nazis' in search of ancient and powerful relics). Doveday's delivery sounds like Rick Moranis in Ghostbusters. Whytecrag paraphrases from The Tempest ("brave new world that has such creatures in it!")
Fluffs: During his final transformation CP seems to mention escargot.
Goofs: The reckoning that the Citadel is three times the size of St Paul's is sort of useful, we suppose, if you're familiar with the landmark as Ace, Hex and the Doctor are. But why would the (American) Institute use the same reference, as Corbin suggests? Why not, for example, say it's nearly twice the size of the Washington Monument (as it is)?
Penguins. In the Northern Hemisphere? Yes, of course it's a reference to At the Mountains of Madness (set in Antarctica!), but the lack of in-story justification does grate. Did the Karnus-koi ram-raid a zoo while bingeing, maybe?
Dialogue Triumphs: "You're not actually a real doctor, are you Ace?" "No. but I'm very good at second opinions."
Hex on his imminent sacrifice: "You don't want us to wear white or something? 'Cause I gotta say, you might have missed the boat."
"Fear, as the mystics used to say, is the first step toward understanding."
Dialogue Disasters: Whytecrag's rant: "I see their beauty, their purity of form and soul. I searched this degraded mongrolised world for some force which did not compromise its affinity for power. And here at last, thanks to a sympathetic informer in Washington, I have found it!"
Double Entendres: "Where would the pursuit of knowledge be without occasional recourse to a guinea pig?"
"Oh Corbin, we both know you don't have it in you."
Continuity: The Karnas-koi were vast, winged creatures living on Earth up to six million years ago (and therefore around two million years before the Australopithecine divergence), but their society collapsed and four of its mightiest went into a self-induced sleep within their Citadel as it sank beneath the waves. On the ocean floor they have been contained by a 'sleeping chant' and their release requires two keys - one is of crystal and opens the surface Citadel, the other is golden and opens the ocean floor and the centre of the universe, finally and fully releasing the Lurkers. Their language, according to the Doctor "predates history [and is a] language of the Old Time."
The Citadel is also made of alien material, located in a valley on a recently-risen island off the north Alaska coastline, swathed in a mist of its own creation - the island appeared four years, three months and six days ago. The Citadel measures three times the height and size of St Paul's cathedral [i.e. around 1095 feet]; its door is sixty feet high and patterned like rows of twisted teeth layered end on end, and its whole structure is said to be spun like a cocoon by the culture inside. Within natural phosphorescence illuminates its walls, which have fish-like eyes and spiral-shaped ears set between them, with mouths ready for human blood. The Citadel itself is organic, incorporating membrane walkways and moving walls with concealed spines - intravenous needlepoints to pacify the Lurkers within if they are woken too early. Remains inside the Citadel include those of walruses, whales, giant penguins (see: Goofs) and humans. The key is a living thing itself, resembling a crystalline spike with 'spokes' at its end, like a broken umbrella which shrinks over the arm of its user - it is not designed for human ball and socket limbs. There is a map of their home galaxy, located on the far reaches of the Universe, at the bottom of their chamber.
According to 'ancient' texts "three Lurkers dream in the vaults, with the Fourth Dreamer elsewhere [and] in the shape of a man." This form is Clarence Penrose ('CP') Doveday, writer of strange fiction in such titles as Shuddersome Tales and Uncanny Yarns. CP is an advance envoy created in the Citadel as a normal human being, released to the surface world with the crystal key and entrusted to reconnoitre Earth in advance of the Lurkers' "second shot" at conquest. However, the Doctor surmises that Doveday was created "too human" and the conflict between his alien mind and human nature resulted in psychotic episodes of hallucination and recurrent blackouts, with the results of his lucid dreaming forming the majority of his published works. Seeking the help of New York psychologist Doctor Freya Gabriel he underwent hypnotic regression therapy where Gabriel and her recent colleague Professor August Corbin unravelled his true origins and created false human memories to subdue it. Realising that Doveday's Lurker self was in telepathic contact with his kind - a 'hive mind' as the Doctor describes it, they attempted to use CP to subdue the awakening sleepers, removing him to a new Institute hospital on the island along with every copy of his published writing (the aforementioned 'texts') available.
August Corbin's background interests are described as being in modern unexplained phenomena, and his apparent donor is Emerson Whytecrag III, an extreme right-wing billionaire who was kept under the watchful eye of Washington. Corbin ran the Institute until two months ago, presumably working then with Whytecrag to locate the Citadel, stealing CP's crystal key from the Institute's safe and substituting it with a glass replica. The coordinates to the Citadel are extrapolated from Doveday's short story In the Icy Citadel of the Monstrous Gods
The TARDIS anoraks have 'Yeti cloths' in their pockets (see: Links) and Ace finds a torch in one of her pockets.
The Doctor can crack a safe by listening to the turns of its dial.
Location: An island off the coast of northern Alaska, 1934.
Untelevised Adventures: The Doctor suggests he may have met the Lurkers before.
Links: Dragonfire (the Doctor refers to an ice descent: "I've done this before, though not particularly skilfully") The Angel of Scutari (The TARDIS is still white), The Abominable Snowmen (Yeti cloths). Hex refers to having seen the Doctor's death and later return from the dead (A Death in the Family.)
The Bottom Line: "Nutters with guns. Now that I do understand."
Combining the worlds of Doctor Who with those of H.P. Lovecraft (or a recognisable facsimile of him) should surprise nobody expecting this result - nods, winks, a surplus of histrionics and funny voices. Given that a good number of Lovecraft riffs tend towards a dense core of atmosphere with a thin veneer of plot, Lurkers is a reliable attempt, and Michael Brandon's Doveday a sympathetic analogue to the troubled writer. The only real sore thumb in this story then is the Doctor, who offers the explanations, keeps the white coats busy, but strays too far away from the action to matter. Coming so soon after A Death in the Family, this story can only suffer by comparison.