This one had a higher onscreen body count than "the Terminator", as I recall.  It's a real nasty piece of work from a head writer who wanted to do stories about grim space mercenaries but had to keep finding ways to shoehorn the "Doctor" character into them.  See also "the Revelation of the Daleks".

In Aliens, the robot guns scene is important for establishing why the creatures choose to sneak into the complex via the ceiling instead of just using the doors.  And the scene where Ripley finds out her daughter died of old age nicely establishes why she's so protective of Newt.  If I could keep those and jettison…

In Aliens, the robot guns scene is important for establishing why the creatures choose to sneak into the complex via the ceiling instead of just using the doors.  And the scene where Ripley finds out her daughter died of old age nicely establishes why she's so protective of Newt.  If I could keep those and jettison…

This story is very typical of the Second Doctor era:  a future or near-present human society with some glaringly obvious fault is attacked by alien invaders.  See also "The Power of the Daleks", "The Moonbase", "The Macra Terror", "The Ice Warriors", "Fury from the Deep", "The Moonbase", "The Dominators", "The…

It's also interesting to note that this episode dates to a period before the Doctor as his as-yet-unnamed civilization weren't established to be aliens; they were still being treated as human beings from some unknown time period.  So, the Monk's interest in Earth history isn't treated as anything that needs to be…

This episode is full of great, subtle details.  Why does Mad Pierrot flinch in the final confrontation with Spike?  Because he sees that Spike's eyes are two different colors - like the cat's eyes in the research facility.  All those intricacies make this a treat to watch.

Doohan must be a master mechanic for being able to repair the space shuttle Columbia, which he uses to rescue Spike.

Although now we would think of a forced regeneration as a death sentence for the second Doctor, at this point in the series the concept of "regeneration" as a sort of death and rebirth hadn't been invented yet.  That wouldn't come until "Planet of the Spiders".  The transformation of the first Doctor to the second was…