Ian, Barbara and Susan spend most of the series either arrested and confined in The Conciergerie prison or escaped therefrom and working with a set of spies.
Along the way the Doctor has several conversations with Robespierre, and Ian and Barbara operate clandestinely as bartenders serving the meeting at which Napoleon was offered the opportunity to move from the Military into Politics such that he later became Emporer. They also facilitated the downfall of Robespierre.
It was interesting that they did the French Revolution when in the initial episode, An Unearthly Child, they had Susan pick up a book on the French Revolution, start leafing through it, and then exclaim "That's not True!" as if she knew, from firsthand knowledge, the history better than the author of the history book. Perhaps the Doctor and Susan had visited the French Revolution before the beginning of the aired episodes. OR... It's known that the producers were reshuffling the episodes up until the last minute and it's possible they'd meant for the French Revolution episodes to be aired before An Unearthly Child. But that doesn't make much sense because An Unearthly Child makes much more sense as the first episode than it does as a middle episode.