Roots: Aladdin ("Your wish is my command")
Continuity: The planet Zaleria was once Spiridon, renamed by its people as a new start after earlier Dalek occupation (see Links). Despite this, the planet has seen a return of the Daleks , once more seeking to overcome the limitations of light wave sickness caused by their experimentation into invisibility. Zaleria has a completely unblemished record of co-operation under Dalek rule (half the ships on this planet are burnt-out wrecks - it seems the Daleks were relaxed enough to invade Zaleria without decent hardware). The Daleks are a known chapter in its people's history, though it's not widely known (but is written in their sacred texts) that the Zalerians once worked for the Daleks, and only the Elders are aware of the army in the ice caverns.
The Zalerian secret of invisibility is confirmed here as a natural trait of the former Spiridonians. In a further attempt to conceal their identity and abilities from outside aggressors the previous generation subjected themselves to many painful forms of treatment, including the ingestion of pigmented chemical substances and nutrients that decayed their non-reflective cellular properties. It is now an essential part of their diet, but despite it the Zalerians' visibility is merely skin-deep and their genetic coding is constantly resisting the diet's effects.
The Daleks' drilling scans reveal one million and one hundred thousand Dalek units in over twenty chambers submerged under approximately thirty thousand cubic gigathrons of molten ice beneath a ten gigathron skin of frozen ice (this is many times greater than the ten thousand the Doctor believed were entombed when he visited in his third body).The Daleks never fully mastered invisibility because the massive amounts of energy they had to expend to maintain it caused light-wave sickness. They use Ogrons as security forces.
The Doctor has recently overheard Dalek command discussing a rebellion on Yaldos (see: Links). He is aware of Kalendorff and the Knights of Velyshaa's mission with Earth. The dimensional stasis of the TARDIS neutralises his light-wave sickness, but cellular paralysis prevents his regeneration - a self-induced coma fully heals him, however.
Links: This story is set between episodes one and two of the events of the first series of Dalek Empire. Planet of the Daleks and Frontier in Space (Ogrons). The waking Doctor calls out for Mel, Ace and Hex.
Location: Spiridon aka Zaleria during the time of the Daleks' second great occupation.
The Bottom Line: "Seismic probing will commence!"
With story beats straight out of its Pertwee predecessors Return of the Daleks could easily be seen as a rehash (even the name is long in the tooth). What improves it is the solid backstory of the Dalek Empire universe, and the inclusion of a 'last days' Seventh Doctor as a surprising bridging character between the two series.