There are many places that most of us can never see: places that are sheltered, locked away, cordoned off from the outside world.
But to the Doctor, and those who travel with him in his TARDIS, there is no such thing as a locked door. Anywhere in space and time is open to them to visit - even if sometimes it might be better to leave such places well alone.
Steel Skies is a collection of stories based in enclosed and artificial environments: places constructed to keep the dangers of the universe outside, perhaps, or to keep their inhabitants locked in. It is divided into four sections, each exploring a different kind of confinement:
Section One, Flight, comprises four tales of travellers who left their homes for far-away destinations - to explore, to start a new life, or to fight for the survival of their species.
Section Two, Frontiers, explores the corridors, living quarters and ventilation shafts of four futuristic environments - designed to shelter men, women and children from harsh natural forces, or from the threat of nuclear war.
Section Three, Incarceration, tells four stories of punishment and imprisonment, from San Francisco's infamous Alcatraz, to the cage of a flightless angel in the dilapidated ruins of Heaven.
Section Four, Isolation, deals with the loneliness and despair of being cut off from the world outside, by physical or mental incapacity, by the ravages of war, or caught between destinations aboard the TARDIS itself.
A recurring theme in all four sections is the effect of the Doctor's arrival in these enclosed environments - sometimes positive, sometimes less so