Right place, wrong time. While the Doctor finally succeeds in getting Ian and Barbara back to London, he overshoots the date of their original departure by about 200 years or so. In this era, the Daleks have already managed to invade and conquer the Earth. While some resistance fighters do persist, the bulk of the remaining human population have either been forced to work in the Dalek's deep mines or they have been converted into remote controlled slaves (aka Robomen). When the group gets separated after their arrival, Barbara and Susan find themselves travelling through London's sewers with a group of resistance fighters while Ian and the Doctor fall into and out of the Dalek's custody. The serial ends with the Doctor leaving Susan behind on Earth, in a move that is somewhat cold (she is his granddaughter and Earth isn't exactly in great shape when he leaves) but would prove to be only the first in a never-ending series of cast changes.
Rating: One day, I shall come back. Yes, I shall come back%