The one where the true identity of James Stirling is revealed...
After a really awkward staring contest in which the actors have to look at one another silently as the title and writer captions fade in and out, we finally get to know who James Stirling - the English spy - is... it's Lemaitre! After all that, Lemaitre's scheming and eavesdropping was in order to make friendly contact with Ian, in order to learn Webster's dying message!
We then gets lots and lots of explanatory dialogue and info-dumping, setting the scene for what is an episode packed full of incident but also quite heavy on talking politics. If you didn't know anything about the political history of France before watching The Reign of Terror, you'll be an expert now! I do love the Doctor's focused insistence on getting his granddaughter out of Conciergerie however, and his frustration at having to delay Susan's release. As he says to Lemaitre: "Very well, if you must tell your story then get on with it."