The very good arcs follow one another, as evidenced by The Aztecs.
In addition to being fairly faithful to reality (at least, what was thought of it in the 60s), we have here for the first time the idea of ​​HAVING to not change the course of events, with a Barbara as ashamed of "corrupting" a man who believes in her, who has confidence, just for her own will, but also and above all disconcerted by not being able to help this civilization which is according to her too primitive.
She realizes in certain points what the Doctor feels when them sees us, poor primitive humans according to him.
I also like the idea of ​​this search for an episode almost exclusively focused on Barbara: Ian fights people, okay, but in the end, everything is brought back to Barbara, and in such a logical way that the episode becomes very good.
Really, episode in 6/10.