Forest of the Dead

The team flees the planet-sized Library as the Vashta Nerada close in. Some crew members are consumed by the swarms, and the Vashta Nerada seem to animate the survivors' space suits. The escape marks the start of a dangerous trek through the Library's corridors.

Lux explains that the Library houses a giant computer core built by his grandfather for Charlotte to preserve her mind among humankind's books. The plan was to let her live on despite her terminal illness. The Doctor realizes the Library is more than a building—it is a conduit for preserved minds.

The Doctor learns that Charlotte's mind has struggled since thousands of patrons were transferred into the core a century ago during the Vashta Nerada attack. The mind's burden explains the lingering tragedy inside the core. This backstory foreshadows River's actions and the present danger.

Within a simulated, contemporary Earth village, Donna is tended by Dr Moon, an avatar of the virus-checker program, and she meets Lee, whom she eventually marries. The simulation mirrors human life but masks the Library's hidden danger. The sequence shows how Donna's consciousness is living inside the Library's constructed memories.

Donna becomes aware that time is skipping and Miss Evangelista warns her that she is in a simulation. The revelation shatters her sense of reality and creates urgency to understand what is real. Her growing awareness connects the Earth village to the Library's larger crisis.

Back in the core, the Doctor learns the Vashta Nerada claim ownership of the Library. They reveal their forests were used to forge the Library's books, making the Library their territory. They grant the Doctor one day to free the trapped minds, otherwise the Library will belong to them.

The Doctor prepares to hook himself to the terminal to provide the extra memory needed to download everyone, a move that could kill him. River interrupts, setting in motion a plan that hinges on her taking his place. The tense moment tests their bond and the cost of saving others.

River knocks the Doctor out and takes his place, insisting that his death now would prevent her meeting him in her own past due to their mismatched timelines. The swap preserves River's future and allows the rescue operation to proceed. The act changes the course of both their lives.

The patrons stored inside the computer rematerialize on the Library's surface and teleport away to safety. Lee cannot call out to Donna as he leaves, marking a poignant farewell. The core's evacuation marks a turning point in the crisis.

The Doctor and Donna leave behind River's diary and sonic screwdriver. He finds a data recorder inside the mechanism that preserves River's thought pattern and saves it to the core. River's pattern becomes embedded in the Library's memory, ensuring her consciousness persists.

River wakes up in the Earth simulation and is greeted by Charlotte and River's teammates who had fallen to the Vashta Nerada. Charlotte assures her that the simulation is now a good place where River will be safe. The scene ends on a hopeful note for River's future within the simulation.