The Doctor has left Susan behind on a future Earth, recovering from Dalek occupation. He appears to be even more distracted than usual. And then the TARDIS lands on the planet Dido, a world the Doctor visited once before. There, he, Ian and Barbara find the wreck of downed space ship and the two survivors who are being terrorized by Koquilion, an inhabitant of the planet.
This story sets up the introduction of Vicki, as played by Maureen O'Brien. She will join the TARDIS and while the producers were looking for her to be a replacement for Susan, she is clearly much more than that. Take a listen and see if you agree with the assessment of the episode overall and where it places in the ranking.