In The Library

An enigmatic message, on The Doctor’s psychic paper, sends The Doctor and Donna Noble to a planet-sized library in the 51st century - simply called 'The Library'. They arrive to find the world completely devoid of humanoid life, though the Library's computers claim over 'a million, million lifeforms' exist.

As they investigate they find a Node, an information drone with a donated human face attached for communication, which warns them to count the shadows. As they try to search for answers, they are joined by a team of explorers, led by archaeologist Professor River Song, who have come to ascertain why The Library was sealed off a century earlier and the meaning of The Library's final communication, which states that 4,022 people were saved yet none survived.

Professor River Song seems to already know The Doctor – despite The Doctor having no knowledge about her. Even more strangely is she has a diary with a cover matching the TARDIS, and even possesses a sonic screwdriver. When questioned by The Doctor she will only admit that she will know him in his relative future, refusing to admit more for fear of 'spoilers'.

Professor River Song

Joining forces The Doctor, and the mysterious Professor River Song, investigate the mystery of The Library. But the shadows are alive with a flesh-consuming intelligence... and somewhere, a little girl believes that all of these events are playing out in her mind.

The Doctor soon deduces that the shadows are occupied by the Vashta Nerada, microscopic carnivorous creatures that use shadows to hunt and latch onto their prey. Then one-by-one the team of explorers are stripped to the bone inside their space suites and are turned into short-lived 'Data Ghosts' of that person's consciousness.

The Exploration Team

Fearing for Donna’s life The Doctor attempts to teleport his companion back to the TARDIS so that he can concentrate on trying to lead Professor River Song and the rest of her team to safety, but something goes wrong in the teleport and Donna fails to materialise properly. As they race away from the possessed suit of Pilot Dave, The Doctor is horrified to find a Node with Donna's face on it, which claims that Donna has left the Library and has been 'saved'...

The Doctor is about to face secrets from his own future. What or who is CAL? Will The Doctor and Donna be reunited and can the library’s population be saved? Will The Doctor find a way to stop the Vashta Nerada and rescue his companion?

Curiously, the Library's operations seem to be tied to the imagination of a young girl in the 21st century; she sees The Doctor and Donna through the eyes of a security camera when they first break into central room, the exploration team appear on her television as The Doctor attempts to hack the Library computers, and books fly from the shelves when she fiddles with the television's remote. The girl is under the observation of Dr. Moon, a child psychologist, at the request of her dad, but Dr. Moon insists to the girl that what she imagines is the real world, not her so-called home, even revealing knowledge of The Doctor and Donna.

Professor River Song and The Doctor

Unknown to The Doctor, Donna finds herself in an alternate reality being treated by Dr Moon and with a husband and two children. Donna’s world, living in a home named 'CAL' is an idyllic one, despite time jumping ahead at each thought in her mind – that is until a strange cloaked figure in black makes Donna realise that things are not as idyllic as she first thought, especially when she discovers that the cloaked figure is in fact the member of Professor River Song’s exploration team who she tried to help before she was attacked by the Vashta Nerada. Meanwhile, the little girl watches both The Doctor and Donna by switching channels on her television.

The Doctor then learns that The library is the Vashta Nerada’s "forest"; the paper of countless books in the library were made from trees filled with Vashta Nerada spores, from which they hatched. The Doctor also discovers the meaning of The Library's final communication – the '4,022 saved' did not mean they were rescued, but that their teleport patterns were saved to The Library's hard drive to protect them when the Vashta Nerada started to swarm. He also learns that CAL is an acronym for the name Charlotte Abigail Lux – the name of the little girl and who is a familiy member of Strackman Lux - who is funding Professor River Song’s exploration team.

Charlotte, it seams, was uploaded into the computer as a child because she was dying so that she could live forever with the sum total of human knowledge to pass the time. However, storing the patterns of 4022 unique people has filled her computer core, and is preventing normal operations. The Doctor deduces that the only way to set things right is to reintegrate them back into the library. As CAL cannot do this alone, The Doctor prepares to wire his own mind into the system as extra memory, even though it will surely kill him. Professor River Song, realising the danger The Doctor is putting himself in and unwilling to let him die - which would rewrite history and erase their time together, knocks him out and takes his place, rescuing those trapped in the computer at the cost of her life instead of his.

As the rescued humans are teleported home, Donna is reunited with The Doctor and the both of them return to the TARDIS, but before they depart The Doctor suddenly realises the reason why his future self gave Professor River Song a sonic screwdriver - it holds a communication device with a Data Ghost – thus enabling him to bring her back to life inside the computer.