Dress Rehearsal is part 9 of 13 and yet is the first to feature all three "Eagles" within the same episode. Even then the focus is on Russia, with Kaiser Wilhelm and Emperor Franz Josef limited to a one scene cameo each, and none of them appearing together.
The main character of the episode is Russia's egotistical foreign minister Baron Isvolsky, excellently played by Peter "Denethor" Vaughan. He has a plan to force the Ottoman Empire to open the Bosphorus strait to the Russian fleet by getting all the other countries of Europe to side with Russia against the Ottomans.
The Tsar has expressed his wish for this, but the new prime minister Stolypin (Frank Middlemass, a Shakespearean actor who I recognise from playing the Fool in King Lear and Cardinal Beaufort in the Henry vi plays, as well as the Master of Baillie College in Yes Minister) predicts that Russia will have to give away more in concessions than they will gain from it.
Most of the episode sees Isvolsky proving Stolypin right, as he travels Europe meeting important minor characters representing their countries, including French foreign minister Clemenceau, played by John "Li H'sen Chang" Bennett (proving he can play a Frenchmanny just as easily as a Chinamanny).
The trouble is that Isvolsky is not nearly as clever as he thinks he is, and it is with the Austrian foreign minister Aehrenthal that he more than meets his match - agreeing that Russia will recognise an Austrian annexation of Bosnia, which would mean betraying Russia's supposed ally Serbia, and foolishly doing so in writing without the agreement of the Tsar or Stolypin.
Then when Austria annexes Bosnia straight away, before Isvolsky is finished making all his deals, Russia has effectively given away Bosnia for nothing. This leads to a great scene where the German ambassador makes it plain to Isvolsky how outmaneuvered he has been, and that he has to go along with what the Germans and Austrians want if he does not want the secret deal made known, or else shoot himself. The First World War only fails to start several years early when Russia backs down.
In the end Britain's King Edward vii asks his foreign minister Grey if all this politicking and backstabbing has been worth it for anyone - all that has been achieved is the change of status of Bosnia from occupied puppet to an annexed part of the Austrian empire (it sounds to me like he's been playing Civilization V). But it has firmed up the alliances between Britain, France and Russia, and between Germany and Austria. The two mannys realise that if a war happens, it will be between these two sides.