This anime is based on a series of light novels. The story takes place after World War One and is about mysterious collections of magical books that contain secret, forbidden knowledge. The collections are guarded by equally mysterious girls and their “key keepers.” Our main protagonist is a young man named Hugh Anthony Disward, who discovers that his grandfather left him his mansion and book collection when he died. His grandfather also left him a mysterious little girl named Dalian who it turns out, is the custodian of one of these mystic book collections.
The first episode opens with a mysterious place filled with water and spiral staircases made of light. We see a young boy and a white haired girl dressed in white. The girl is reading a book. This is a very dreamlike scene and the conversation is very cute but also slightly creepy. We learn that this space without any walls, floor, or ceiling has many names, among them the Labyrinth Library. The boy decides to befriend the young girl, and then we go to credits.
When we get back from credits, we’re introduced to our Main Protagonist, Hugh. His grandfather had been murdered by a burglar, and Hugh has inherited his home. The grandfather had been a “bibliomaniac” with an intense passion for books. (Both collecting and reading.) We learn that the grandfather’s obsessions and general attitudes had caused the family to avoid him, and there seems to be some family legend that the grandfather kept “books of dark and forbidden knowledge.”
We see something fairly interesting when he arrives the mansion. He initially tries to open the door with a very elaborate golden key. However, the key is too big, so he ends up using a much smaller, ordinary looking key. After some exploration of the mansion, he sees that the book shelves are empty, and that there are also drag marks in the dust on the floor. Following the trail down to the cellar, Hugh discovers a young girl with a very imperious manner, and the location of all the books that had previously been on the shelves.
During the conversation between the girl and Hugh, we learn that Our Protagonist is looking for the “Mystical Archives of Dantalian,” and this is the primary reason that he had decided to come to his grandfather’s estate. (He inherited both the mansion, and the guardianship of the “archive.”) He was also instructed to look after Dalian, whom he at first assumes was some kind of pet. The little girl however informs him that she’s Dalian and clobbers him for referring to her as a “pet.”
Under calmer circumstances, we learn that Huey was a pilot during the war. Huey talks about his quest for the library, and Dalian implies that she is not human. Huey is disappointed that his grandfather’s collection wasn’t the mystical archive he was looking for, but Dalian tells him that he doesn’t know what he’s talking about. She tells him a story about a story called “The World in the Gourd,” and tells him that the mystical library he’s looking for operates under a similar situation. Much to Dalian’s annoyance, he doesn’t understand what she’s getting at. (A Mystical library might be located in another dimension and take up no space in the “real world.”)
The conversation is interrupted by a sudden invitation to the home of a Mr. Henry Conrad. Dalian invites herself along for the ride to Mr. Conrad’s house. After some questioning back and forth, Dalian reveals that Conrad is the man who killed Huey’s grandfather. Huey in return reveals that there were some rumors to that effect, and that Huey was investigating to see if those rumors might be true.
It turns out that Mr. Conrad and Huey’s grandfather were rivals of a sort, and Mr. Conrad stole a book from the grandfather’s collection. Said book turns out to be something extremely dangerous. There are books known as “Phantom Books” which if read under the right circumstances, may impart knowledge, or if you open them under the wrong circumstances, might kill you. The book Mr. Conrad stole was a magical pop-up book and his mansion is now filled with homicidal circus acts and animals.
Huey states that he is somewhat skeptical of this story. (Then how would you explain the presence of creatures that explode into paper and paint when you shoot them, Huey? I mean, really.) Then they get attacked by a dragon, and there is some fairly humorous back chat between Huey and Dalian as they dodge the dragon’s fiery breath.
The situation looks pretty grim, and Huey wants Dalian to run for cover while he acts as a decoy so she can escape. Dalian however abruptly reveals that with the help of Huey’s key, she’ll be able to help him fight the dragon and put the book out of commission. She tells him that she acknowledges him as her “keykeeper” and tells him to read the words on the key. Then we get a “magical girl transformation” of sorts and Huey gets a book from the archive (which is apparently Dalian) and uses it against the dragon. (He does this by reading it. As he does, we see some flashbacks from Huey’s past.)
After they destroy the dragon, they head into Mr. Conrad’s study where they find the book, and the remains of Mr. Conrad. The book is sealed, and we end with a conversation in the key of “there are some things man was not meant to know.”