The very short version of it... In the world of Full Metal Alchemist, trying to use alchemy on living people turns out badly, always. We spend several episodes of the main characters staying with and learning from an alchemist, and getting to know him and his sweet daughter and her dog. Once everyone had been charmed by these two, alchemy was used to merge the girl and the dog and like I said... It didn't turn out well. In a lot of ways, it's a defining moment for the main characters, and represented a lot, both in their lives, and in the tone of the story. As such, for fans of the show it is a huge emotional moment. In a very real sense, that storyline is when the main characters stopped being children and had to face some very hard realities about their world and their craft. Not that they hadn't already....