Saito and Louise finally stop dancing around what they actually are to each other. The show stops using their relationship as setup for comedic interruptions and makes it the thing that everything else breaks against. By the final arc, every obstacle exists to test whether they'll choose each other over everything else, and the season doesn't flinch from making you watch that choice get harder each time.

Saito and Louise stand on the edge of something bigger than themselves. Their bond has grown deeper, and they feel ready to take the next step in their relationship, but the world has other plans. A secret order from Queen Henrietta sends them to the Holy City of Romalia, with Tiffania coming along, and the pope names both Louise and Tiffania holy maidens. There they face King Joseph with the help of two others who wield the power of the void, while an ancient evil begins to stir in the shadows. Interruptions from friends and foes alike put their love to the test. This fourth and final season of The Familiar of Zero mixes action, fantasy, comedy and romance as the pair meet their last challenges together.