Davison isn't my favorite Doctor, but this is one of his better episodes, although I agree the Black Guardian seems kind of peripheral here. When a bit of his sabotage brings the outside universe encroaching into the Tardis, it instinctively fuses with a nearby vessel that turns out to be a sort of hospital ship on route to planet Terminus. But are the patients being taken there to be cured, or merely to die? From there the stakes rapidly escalate, with the entire universe (of which Terminus mysteriously happens to be in the exact center) slated for destruction if the Doctor and his companions can't disarm the planet's self-destruct sequence in time. Those companions include miniskirted techno-chick Nyssa, teaming up for one last time with sharp-tongued Aussie airline hostess Tegan before being (foolishly) given the boot by the show's producers. An array of quirky supporting characters and subplots round out this above-average Who adventure.