Plot summary
Robert arrives in France and Queen Mary agrees to marry him. She and Gideon discover that although the Vatican wish to put Queen Mary on the English throne, they are going to replace her with a distant Catholic cousin, Joseph Tudor. Gideon kidnaps Joseph but is arrested by Archbishop Ridolfi before he can expose the plot, so he passes the task of warning Elizabeth to Robert. Robert returns to England with Joseph and is hailed a hero. Queen Elizabeth executes Joseph and declares that she will have no successor; Robert and Queen Mary's engagement is cancelled as well.
Queen Mary arranges for Gideon to return to England to save him from the Vatican's retribution, and the pair bid their goodbyes. Queen Catherine, with Bash and Christophe's help, learns that the Red Knights are a movement to destroy the House of Valois, and were behind the attempt to frame her for poisoning King Francis. Queen Catherine reconciles with Lord Narcisse in her preparation to fight back. Greer breaks ties with Ellen and her husband because of their duplicity. Princess Claude offers to financially help Leith start a new business, and he accepts.
Credits
Adapted from Wikipedia contributors, Reign season 3. HTML formatting and reference markers removed.