"Forest of the Dead" is the ninth episode of the fourth series of British science fiction television series Doctor Who. It was first broadcast by BBC One on 7 June 2008. It is the second of a two-part story, following "Silence in the Library". The events are set in the 51st century.

The Doctor, River Song, Strackman Lux, and the remainder of their team flee the Vashta Nerada, microscopic carnivorous creatures that are thriving in the shadows of a planet-sized Library. Other team members are consumed by the Vashta Nerada, their suits and skeletons animated by the creatures who then chase down the survivors. 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 CAL represents Charlotte's mind, struggling to cope after "saving" the thousands of patrons by transferring their consciousnesses to the computer core when the Vashta Nerada attacked. Even the virus checker program housed in the Library's moon, nicknamed "Doctor Moon", is having difficulties helping CAL maintain control.

Within the simulation, an apparently normal contemporary Earth village, a trapped Donna Noble is tended to by Dr. Moon and introduced to Lee, whom she marries and with whom she raises two children. Aware that time seems to be skipping in the simulation, Donna is alerted to its computerized nature by Miss Evangelista, one of River's team members killed by the Vashta Nerada. Her corrupted consciousness makes her appear deformed, but allows her to see the reality for what it is. Donna returns home questioning her reality, which results in her children disappearing.

The Doctor, River and the others descend to the core. The Doctor attempts to talk with the Vashta Nerada. He learns that their home, forests from a different planet, were used to create the books of the Library. They now claim the Library as their own. The Doctor demands that he be allowed to free the people trapped in the computer core, including Donna, after which the Library will belong to the Vashta Nerada. The creatures agree and permit them one day. The Doctor prepares to hook himself to the computer terminal, aware this will likely kill him. River knocks him out, handcuffs him to a pillar, and takes his place. The Doctor tries to stop her, but River insists that his death now would prevent her meeting him in her own past, and whispers something to him that astounds him. River refuses to tell him who she is before activating the interface, much to the Doctor's anguish. As River initiates the connection, the simulation starts to fall apart. Donna desperately holds onto Lee as the simulation fades to white.

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