Charlotte Abigail Lux, also known as CAL, was a human who later became a node. Once a healthy young girl, she was afflicted by a deadly malady that would have ended her life at a young age, had the decision not been made to liberate her from her failing body and transplant her consciousness into a new form of life.
Charlotte was the youngest daughter of Felman Lux, Strackman Lux's grandfather. Felman Lux built his dying daughter the Library, its hard drive to contain and embody her living mind and a Doctor Moon to maintain the Library and her spiritual well-being. The library contained all of human history to keep Charlotte entertained. She loved books more than anything and her father gave her all of them. Felman Lux also made sure that nobody outside of his family knew of the main command node's (Charlotte) true nature; he wished for her to live in peace and not be treated like some kind of freak show. (TV: Forest of the Dead)
When the Vashta Nerada began to hatch, CAL became aware of them. She tried to warn the people in the Library of the threat. The warning failed when the Vashta Nerada devoured the head librarian. (TV: Silence in the Library) She tried to evacuate the people herself through the teleporters, but because the Library was now infested with Vashta Nerada, quarantine seemed hopeless. Instead of saving them, CAL "saved" the 4022 people to her hard drive, putting them in a dream world to keep their minds occupied until rescue came. She then put herself into sleep mode because of the stress of having so many minds in her own. She forgot her true nature. She was watched over by the Doctor Moon that continued to function despite the disaster. (TV: Forest of the Dead)
In her perpetual dream, she described to the Doctor Moon a world she saw when she closed her eyes. It was the Library, where she encountered the Tenth Doctor and Donna. They came across a levitating wooden sphere, which, with a security camera, was her means of communicating with the world. In the dream world, she contacted the Doctor through her television screen. They held a brief conversation which was ended when the technology malfunctioned.
Shortly after this meeting with the Doctor, the Doctor Moon talked with her. He shocked her by telling her that the people inside "her" Library were very real, that her "real life" was a lie and they needed her help. She interrupted the Doctor's teleporting of his companion to the TARDIS (TV: Silence in the Library) and unnecessarily "saved" Donna Noble, uploading her to CAL's hard drive and turning her into a node, with her mind free to experience a dream life similar to that which CAL lived. She did the same with members of River Song's crew killed by the Vashta Nerada. She kept watch over the happenings in the Library through her TV and in her dream world. CAL witnessed the meeting between Donna and Miss Evangelista during which her memory began to return, as showcased by the fact that she panicked when Miss Evangelista started to tell Donna about the world being a virtual reality. Learning the truth of her "dream" from the conversation, CAL used her control over the world to injure one of Donna's children to end the conversation and began to go insane from everything, shutting off her father and Doctor Moon when they tried to help and inadvertently activating the Library's self-destruct. Unable to save herself, CAL began pleading for help.
The Doctor and River Song realised what had happened to Donna had also happened to the 4022 "saved" by CAL a century earlier. Reaching the data core, the Doctor found CAL to be in permanent sleep mode and couldn't wake her. Strackman Lux finally told him the truth about who CAL was and the Doctor attempted to teleport out everyone trapped inside CAL which would cause her to reset and the self-destruct to end. However, CAL no longer had the memory capacity to do so causing River Song to sacrifice herself to use her own brain space to allow everyone to be teleported out of CAL. Following the freeing of everyone from CAL, the self-destruct ended and the Vashta Nerada allowed everyone to leave per a deal the Doctor made with them, completing what CAL had attempted to do a century earlier.
Following the freedom of everyone, the Doctor realised his future self had given River a sonic screwdriver that had a neural relay that would allow him to upload her data ghost to CAL. The Doctor succeeded in uploading River while CAL's Command Node smiled at him. He then repaired her data core, returning CAL to normal completely. In return, CAL provided a final home for the consciousnesses of River and her crew. CAL's sanity was restored and her computer avatar was adopted by River Song, along with Donna's children. River would tell them the story of the Doctor and how much good he did, reading to CAL and the other children from her diary at night. (TV: Forest of the Dead)