Picking up where the debut serial left off the doctor, Ian,Barbara and Susan find themselves trapped on a planet high in radiation. Using the ruse of the TARDIS's need for Mercury as a ruse to explore this planet they find a domed city and evidence of an atomic war. While exploring this city Barbara is the first to be tormented by an army tank like creature referring to itself as a Dalek. While off to the TARDIS to retrieve anti radiation drugs Susan encounters the Thals,a now pacifist race who had once fought a war with the people who eventually evolved into the Daleks. Upon meeting up with Ian,Barbara and the doctor it is Ian who convinces the Thals to set aside their pacifist ways to help defeat the cold blooded and merciless Daleks.
The completely militaristic Daleks are ideal adversaries for the doctor. And although they were primarily to have been for the first doctor,they remain the central villain of the program to this day. Why? Aside from their distinction,the Daleks provide an opening for many important sociopolitical points to be made. Ian,while describing the bigotry between the Thals and Daleks as "the dislike of the unlike" is also quick to remind the Thals that,as it was during the very real WWII,there are times when aggressive action is needed to thrawt an enemy that knows no compassion or mercy. As TV science fiction first notable hybrid of the biological and technological the Daleks provide the other characters a window into their own morality. Not to mention the Doctors own questionable act of endangering his companions for the sake of exploration. These factors alone make this a key part of the early years of Doctor Who.