These reviews are based on the UK broadcast of the series, which is several episodes ahead of the US broadcast, so beware of spoilers.
NOTE ABOUT THESE EPISODES:
This story involves two parallel plot threads. While these threads connect in certain areas, they mostly stay separate. To ease the possible confusion, events in Plot Thread A are in plain text, while events in Plot Thread B are in bold.
PART 1 SUMMARY:
A little Girl is under examination by child psychologist Dr. Moon. Her dreams are filled by a vast library. The girl is frightened when intruders enter her library; these uninvited guests are the Doctor and Donna.
The Doctor and Donna arrive in "The Library", an entire planet in the 51st Century devoted to knowledge with the universe's largest hard drive as it's core. The Library appears deserted, and the Doctor runs a scan for humanoid life; only he and Donna register. When he widens the scan to include any forms of life, the result is 1,000,000,000,000.
They are greeted by a Node, an information drone bearing a human face. The Node replays an urgent message recorded by the head librarian: "Run. For God's sake, run. Nowhere is safe; the Library has sealed itself. We can't... oh, they're here!" The Doctor asks if any other messages were recorded, and the Node reponds, "Count the shadows. For God's sake, remember, if you want to live, count the shadows." The Doctor tells Donna to stay out of the shadows, and admits he didn't just bring them there on a whim; he received a message on his psychic paper, "The Library. Come as soon as you can. X."
The lights begin to go out, and the pair run to keep ahead of the darkness, breaking into a room lit through a glass roof. They encounter a camera drone. The Girl snaps out of her daydream, and the drone switches off. The Doctor probes it with his sonic screwdriver, and the Girl hears the noise and begs for it to stop. The Girl's pleas appear scrolling across a screen on the camera; the Doctor assumes the camera is alive, apologises and leaves it alone. The Girl tells Dr. Moon, "The Library is breached. Others are coming." This message also appears on the camera's screen.
Donna asks a Node what the message means, but the Doctor says it won't have any useful information. She asks why it has a face, and the Node says the face was donated by someone upon their death. Donna is shocked by this, but the Doctor explains that donations of that nature are common in the 51st Century. The Doctor catches Donna before she can step back into a suspicious shadow that seems to be casting itself. He suddenly realizes the significance of the message, "Count the shadows." Donna points out that the shadow on the floor is gone; the Doctor says it isn't gone, it's moved.
The Node repeats the message that others are coming, and a group of spacesuited figures burst in. It is an archaeological expedition led by Professor River Song. Her team includes Proper Dave, the pilot; technicians Anita and Other Dave; Strackman Lux, financial backer of the expedition, who's family also built the Library; and Miss Evangelista, Lux's secretary. The Doctor is unable to convince them to return to their ship and leave, and River says the Library has been sealed for 100 years, so anything dangerous that was inside should be dead after that time. The Doctor orders them to create a safe zone by setting up lights and to be careful not to let their shadows cross. River thanks the Doctor for responding to her message and acts like they've met before, but he doesn't know who she is.
Proper Dave's attempt to access the security protocols causes a telephone dial tone to echo through the room. The Girl tells her father the phone is ringing, but he doesn't hear it; it stops before she can answer. The Doctor makes his own attempt to access the data core, and he and the team appear on the Girl's TV. She recognizes him as one of the people in her Library, but the transmission cuts out.
While trying to regain access, the Doctor notices River's diary, with a binding that looks like the TARDIS. He tries to read it but she takes it away, warning him it contains "spoilers", and that for him to read it is against his rules. The Girl discovers a hidden set of buttons on the TV remote. When she presses them, books in the Library fly off the shelves. The data terminal the Doctor is working on displays "CAL Access Denied." River wonders if the Girl is responsible for the flying books, but the Doctor wonders what her connection to the Library even is. The Doctor asks Lux what CAL is, but Lux refuses to divulge that information since the Doctor did not sign a confidentiality contract.
The Girl presses another button, and a panel in the wall opens up. Only Evangelista sees it, but when she tries to get everyone's attention they ignore her. The Doctor asks what happened the day the Library was sealed, and River tells him there was a transmission, "The lights are going out." The computer then sealed the Library, and it has taken 100 years for Lux's family to decode the seals. A data extract came with the message: "4022 saved. No survivors." Evangelista goes through the open panel on her own into a shadowy room. She screams, and everyone runs through, only to find a skeleton in rags. The skeleton is Evangelista, stripped to the bone in a matter of seconds. Her voice comes through her communication unit, and the Doctor trells Donna she's become a Data Ghost. The comms has a neural relay in it which can hold the pattern of a person's consciousness for a short time after death. After a few minutes, the pattern degrades and she fades away.
Before Dr. Moon leaves, he talks to the Girl alone. He tells her the world around her is a lie, and the Library is the real world, and only she can help save the people who are trapped in it. River tells Donna she knows the Doctor in his personal future. She sent the message calling him to the Library, but it went wrong and a version of him from before the time he met her has answered. Donna asks River that if she knows the Doctor, why doesn't she know her as well, but River is reluctant to answer.
The Doctor reveals why the shadows are so dangerous. He throws a chicken leg into one, and the meat on it vanishes instantly. The shadows are full of Vashta Nerada, microscopic carnivorous life forms that inhabit the dark and can be found on most planets, even Earth - "the dust in sumbeams" - but he's never seen an infestation this large and agressive before. Before the group can head to the nearest exit teleport, the Doctor points out that Proper Dave has two shadows; a swarm of Vashta Nerada has latched onto him. The Doctor seals Dave into his suit, increasing the mesh density in the hope it will keep the Vashta Nerada out, and orders the others to do the same. Before he can hand River his sonic screwdriver, she produces one of her own. The Doctor sends Donna back to the TARDIS via teleport. Before she can fully rematerialize inside the ship, she vanishes with a scream.
Dave's second shadow is gone, but while the Doctor looks for it, Dave suddenly asks, "Who turned out the lights?" The inside of the helmet has gone black, and the suit convulses. Dave continues to repeat things he's already said, and River realizes he's now become a Data Ghost. The suit attacks the Doctor; Dave has been stripped down to his skeleton and the Vashta Nerada are now animating the suit. River uses her sonic screwdriver to deliver a shock to the suit, freeing the Doctor from its grip. Four shadows begin to spread out from the suit as the swarm learn how to move it. The team runs off with the suit chasing after them. The Girl's father calls her to dinner, and she responds, "Donna Noble has been saved."
The team rests among the bookshelves, and the Doctor uses his screwdriver to try and boost the lights. River uses her own, and when the Doctor comments that it looks like his, she tells him he gave it to her. The Doctor realizes he hasn't received a signal informing him that Donna reached the TARDIS. He asks a Node to locate her, and the Node turns to reveal Donna's face, saying, "Donna Noble has left the Library. Donna Nole has been saved." The suit catches up with them, trapping them between it and another swarm of Vashta Nerada...
PART 2 SUMMARY:
River uses her sonic blaster to open up an escape route through the wall as the Girl watches events in the Library on her TV. She switches channels and sees Donna being wheeled into a hospital. Donna is greeted by Dr. Moon, whom she doesn't recognize, but he tells her he's been treating her for two years concerning her delusions about the Doctor and traveling through time and space. They go for a walk, and Donna is confused because they seem to be skipping from one moment right to the next. She meets a man named Lee Macavoy, and in the span of a few minutes they go from dating, to married, to seven years later with kids. During one of Dr. Moon's visist, he fades out and Donna sees the Doctor for a brief moment, but she quickly forgets.
The Doctor and River's team take refuge in another room, and the Doctor scans the shadows for Vahsta Nerada, but something is interfering with the sonic screwdriver. River tosses another chicken leg into the darkness, and its meat is stipped clean, confirming the presence of a live shadow. River tells the Doctor to fix his screwdriver by using the red settings and the dampers, but he tells her it doesn't have either of those; she pulls out her screwdriver and tells him it will one day. Still frustrated by her familiarity with him, he asks who she is. She finally gets him to trust her by whispering something in his ear.
The sonic screwdriver is being affected by a signal from the moon, and Lux explains that the moon is artificial, a "doctor moon", a virus checker that supports the Library's hard drive. When the Doctor tries to find a way around the interference, the screwdriver briefly projects an image of Donna. Before the Doctor can get the projection back, Anita tells them she has two shadows. The Doctor seals her inside her suit and tints the visor, hoping to trick the Vashta Nerada into thinking they've already consumed her. The Doctor notices an extra person in the room; it is Proper Dave's suit, which chases after them again.
The Girl watches this on her TV, then changes the channel back to Donna. She is still confused by the way the world appears to skip from one moment to another, and sees a woman in a black dress and veil outside the house. She receives a letter telling her "The world is wrong," and a request to meet in the park the next day, which it then skips to. Donna meets the veiled lady, who addresses Donna by name and confirms her suspicions about the way time keeps jumping. Donna recognizes her voice, and the lady says she is what is left of Miss Evangelista.
The Doctor sends River, Anita, and Lux on ahead while he hopes to communicate with the Vashta Nerada, but River tells Other Dave to stay with him. The Doctor asks them why they came to the Library when they usually inhabit forests, and the swarm uses the suit's comms unit to tell him they hatched in the Library - "These are our forests." The Doctor realizes the Vashta Nerada spores were in the trees used to make all the books. He notices Other Dave has been repeating the same phrase; the Vashta Nerada have consumed him and animated his suit as well. The Doctor opens a hatch in the floor, dropping through and inching along a support girder.
Donna remebers Evangelista, and that she died. Evangelista tells Donna her memories are still there, but are being suppressed. She points out that every child in the playground is the same; the same boy and girl are repeated over and over. Donna rips off her veil and reveals Evangelista's face, all warped and distorted.
Tipped off by something Anita says, the Doctor finally understands the meaning of the Library's last message, "4022 saved." When the Vashta Nerada attacked, the computer tried to teleport everyone to safety. But with the Library overrun, no place was safe, so the computer saved everyone's teleport patterns, backing them up on the hard drive, literally saving them like data.
Evangelista tells Donna that the world around them is nothing but virtual reality. Her face is distorted because she was just a Data Ghost picked up by the Library's wi-fi. Her pattern was incomplete, therefore her data was jumbled, distorting her body but also raising her intelligence and awareness. The Girl becomes frustrated with Evangelista's interference, and in her anger accidentally deletes her father. She throws the remote on the floor which activates the Library's self-destruct countdown. Dr. Moon tries to reason with the Girl, but she deletes him too. With the support system disabled, Lux says the only way to stop the countdown is to access the hard drive directly. Donna's children begin to question her about their existence, and eventually vanish.
Down in the core, the group hears a child's pleas for help, and Lux opens a door to a sealed chamber. Inside is the Master-Node, with the Girl's face. Lux tells them that she is CAL: Charlotte Abigail Lux, his grandfather's youngest daughter. She was dying, and the Lux family built the Library for her, installing her consciousness as the computer, allowing her to live on in a perfect world with the sum of knowledge to play with. But with 4022 other minds stored inside her, she began to malfunction and forget what she really was. If they can retreive everyone out of the data core, the countdown will stop, but the hard drive doesn't have enough memory left to perform the task, so the Doctor plans to wire his brain in as a boost, even though it will kill him. He tells River and Lux to go back upstairs and prime every data cell they can access for maximum download. River leaves Anita behind to keep an eye on the Doctor, but after they leave he confronts the Vashta Nerada inside Anita's suit; he'd noticed some time back that she only had one shadow again. He tries to secure a deal to let everyone get away and then allow the Vashta Nerada to keep the Library. The suit tries to attack him, but he tells the swarm to look him up in the Library's database; the swarm backs off and gives him one day to get everyone out.
River comes back and knocks the Doctor out, handcuffing him to a girder. When he comes to he sees her wiring herself into the computer in his place. She realizes now that the Doctor always knew how she was going to die; the last time she saw her future Doctor, he took her on one last trip and gave her his screwdriver. The Doctor begins to understand the depth of her relationship to him; she whispered his real name in his ear, something he's never told anyone. River connects her brain to the computer; in the virtual world Donna and Lee are engulfed in blinding whiteness, and as they are separated she promises she will find him.
The 4022 people materialize in main reception, and soon the evacuation of the Library begins. Donna is unable to find Lee, but he spots her just as he teleports out. Donna still wonders about River, how she knew the Doctor from the future but didn't know her. The Doctor says they could look it up in River's diary, but Donna resists the temptation for spoilers. The Doctor leaves River's screwdriver with the diary, but then runs back to it. The future him knew she would die in the Library, and must have had another reason for giving her the screwdriver; he finds a neural relay installed inside it, which is barely hanging on to a Data Ghost of River. He runs back to the computer core and uploads her into the system. She is greeted by CAL and Dr. Moon, and is reunited with Anita, Evangelista, and the Daves. Although their bodies are gone, they can live on forever inside the computer.
