Roots: Norse mythology (Loki, Magni and Modi, Skadi, Freya, Helheim). King Lear, Tegan calls Loki R2-D2 (he's more See Threepio), The Hitchhiker's Guide to the Galaxy (EDGAR as Marvin). The juvenile fiction worlds of the Bronte Sisters (Gondal Prime). The Doctor refers to James "Gentleman Jim" Corbett, US heavyweight boxing champ.
Goofs: "Two crustaceans, with an exoskeleton" (all crustaceans have exoskeletons)
Mark Strickson's accent audibly (and amusingly) slips well below the Equator during some scenes.
What's with the ruins beneath the base? A red herring?
Dialogue Triumphs: "Do you think it could be a Black Guardian trap?" "-Oh, yes Tegan. Causing us to crash land on a distant planet is exactly the sort of thing that might lull us into a false sense of security" "... it could be a double-bluff?"
"Complete system failure. I... I... I knew something bad was going to happen."
"I'm going after Tegan. If I don't come back-" "-yes?" "... be nice to each other."
"The number of times I've heard that. It's almost as if the Universe has its own catchphrase."
Double Entendres: "I'm going to have to perform a materialisation Flip-Flop"
Continuity: Richters Syndrome has re-emerged and spread across the galaxy, more powerful than it was twenty years before. 'Classic' Richter Syndrome may last a couple of weeks, its symptoms include severe cramps followed by intense fear and pain (like being boiled alive). The new strain takes effect in a matter of hours. The Genetech station on Helheim has cured it in its 'present' form, but hadn't done so forty years before. Genetech has exploited the plague before. Its main rival is the independent Bio-developmental Corporation
The Genetech station on Helheim is a top secret facility and is situated far from trade routes and out of transmat range. Three people staff the station - Director Cardell, Biotech Vallis and Security Enforcement Officer Bragg, and assisted by the station's A.I. interface, 'EDGAR' (Encephalic Data Gathering And Research.) All Genetech stations have the same layout. Memories of the staff are wiped to allow total brain use, and stored in a crystal lattice. Below the base are ruins nearly two-thousand years old. There is mention of "nine worlds".
Atmosphere outside the station is a mixture of sulphur and carbon dioxide, producing electrical storms and acid rain, hazardous to both organic and artificial life. Helheim's only indigenous life form, the crustacean Karaktids (a non-intelligent species), are immune to Richter's.
Two days have passed for Tegan and Turlough since Nyssa left them, while fifty years have passed for Nyssa. Seemingly Nyssa ages at a different rate (this contradicts a line from the Doctor in Circular Time: Autumn) 40 years, two months and two days pass before Nyssa and the TARDIS land there. Her ship takes a hyperspace jump. Nyssa developed a cure for Lazar's syndrome fifteen years ago, and since then no further cases have emerged. Nyssa is currently researching it. More than six billion people have died from it (Tegan says that's more than the population of Earth - which it would be, in her time.) Nyssa's servo robot Loki came with her from Terminus and has a self-preservation function.
Turlough recognises maintenance spiders. Tegan's Aunt Vanessa always said 'once a wrong 'un, always a wrong 'un".
The Doctor refers to Arcadian prayer booths.
Untelevised Adventures: The Doctor claims to have met Christopher Columbus.
Links: Tegan says the Doctor should fit the TARDIS with safety belts (Timelash) and mentions her Aunt Vanessa (Logopolis), Terminus and the Black Guardian. Nyssa mentions Brendon School (Mawdryn Undead)
Location: The planet Helheim in the backwaters of "the eastern edge of the [Milky Way?] galaxy."
The Bottom Line: "All crew members must die..."
Spooky and bleak. Jonathan Morris is no stranger to time-twisting stories or built-in paradox mysteries, so his employment of one here is less of a novelty than it would have been in, say, 1982. The choice to age Nyssa is a curious one, and presumably addresses a larger story still to be told (including the obvious connection to the Circular Time stories). The story itself is highly atmospheric, though, with EDGAR and Loki the stand-outs. A great start and welcome return, long overdue.