At the beginning of the first season of Doctor Who, the producers made the short, low-budget story The Edge of Destruction, as filler material. And that story, written so quickly, begin the tradition of suspense and psychological horror in Doctor Who; and also was a turning point in the characterization of The Doctor. At the beginning of the second season, the producers were again, due to external circumstances, forced to write a quick story to introduce a new companion, Vicki, to replace the departed Susan Foreman.
The budget for "The Rescue" was a bit higher than for "The Edge of Destruction", but not by much. It still has a minimal cast (seven actors, two of whom only appear in one scene), some zeerust-y spaceships, and unconvincing monsters. It is the first story in which we gain a new companion, which would be an important aspect of the show for the next five decades. But there is another first in the show, an important point of character development for The Doctor.
"The Rescue" is the first episode in which we see The Doctor full of righteous fury. When he was introduced in An Unearthly Child, he was a sinister figure, and it wasn't until Marco Polo that he softened up, becoming more jocular. It was in The Sensorites that he first became altruistic, going out of his way to help others. But here in "The Rescue", we see him go beyond being a helpful, avuncular figure to being enraged at violence and persecution, becoming the avenger of the weak. And not in a subtle, ironic way: he picks up a heavy object and tries to bludgeon someone he thinks is guilty of genocide. It is a moment that will define the character through five decades and ten different actors, with each of them projecting this side of The Doctor differently. And all of it beginning on a wobbly set thrown together for a filler episode.
For the student of the show, it is interesting to find the little moments that would come to define the central themes of the show hidden in such unexpected places as this story.