 During a respite, Lux explains that the Library was constructed by his grandfather for Lux's aunt, Charlotte Abigail Lux, who was diagnosed with an incurable disease at a young age.Lux's grandfather had a giant computer (CAL, from Charlotte's initials) constructed at the core of the Library to allow Charlotte's mind to live on among the collected works of humankind.The Doctor realises Charlotte's mind is struggling to cope after "saving" the thousands of patrons by transferring their physical forms to the computer core 100 years ago when the Vashta Nerada attacked.