The Doctor (David Tennant) and Donna (Catherine Tate) land on a library the size of a planet in the 51st Century. The Doctor has been summoned, but a scan reveals the Doctor and Donna the only humanoid life signs, surrounded by trillions of nonhuman life signs they can neither see nor hear. But a team of explorers led by Archaeologist River Song (Alex Kingston) and financed by Strackman Lux (Steve Pemberton), whose grandfather built the library, arrive to discover why the library sealed itself off a hundred years ago. It was River Song who summoned the Doctor. She acts like she knows him, but he has never met her. She has a diary with the TARDIS pictured on the cover.

Somehow the library is connected to the mind of a young girl (Eve Newton) on 21st Century Earth. When the Doctor tries to access the library computers, the girl makes books fly off the shelves. She sees the events in the library as TV shows. Her psychiatrist, Doctor Moon (Colin Salmon), tells her that the library in her imagination is real and the real world is a lie. He begs her to save the people in the library.

Lux’s secretary Miss Evangelista (Talulah Riley) is attacked by Vashta Nerada creatures which strip flesh to the bone instantly. The Doctor and Donna discover that the team are wearing comm devices which can store their thoughts for a while after death, and Miss Evangelista continues to talk confusedly until her pattern degrades. The Doctor wonders why the Vashta Nerada that appear as shadows to hunt are so vicious, as they are usually not aggressive. Perhaps it is because they haven’t eaten for a hundred years.

He notices that the pilot, Proper Dave (Harry Peacock), has two shadows. He and River Song (who has a sonic screwdriver) seal him in his spacesuit. The Vashta Nerada still get in, stripping Dave to his skeleton, then they animate his suit to chase after the others. The Doctor tries, but fails, to teleport Donna back to the TARDIS, but he finds an information node with Donna’s face on it which tells him she has left the library and is saved.

This is an extremely creepy episode. The way the spacesuits’ recording devices make the wearers continue talking even after they have died is one of the great horrors of Doctor Who. Kate Winslet was sought to play River Song, but the role went to Alex Kingston from the series ER. She had been a fan of Doctor Who as a child but did not know it was to be a recurring role. She enjoyed playing an action hero in lots of different settings and had no problem with scientific dialogue, having worked with a coach for the medical jargon on ER. Everyone loved her, as did the fans. The reviews were also mostly favorable. Along with the following episode, it was nominated for a Hugo and won the 2009 Constellation award. Writer Steven Moffat told no-one but Alex Kingston who River Song really was. As a result, no-one knew how to act with her, particularly David Tennant, which was exactly what Moffat wanted.

Michael Gambon and Ian McKellen were offered the part of Doctor Moon. In the library are the operating manual of the TARDIS, The French Revolution, A Journal of Impossible Things, The Hitchhiker’s Guide to the Galaxy, Everest in Easy Stages, and The Black Orchid—all of which refer directly to previous episodes. Also, Monty Python’s Big Red Book. The pictures on the wall near the little girl are of a blonde girl and a wolf, referring to Rose Tyler and Bad Wolf. There is also a model of Robby the Robot from Forbidden Planet. She seems very Harry Potter. River Song questions the Doctor about adventures he hasn’t had yet but seem familiar to her. Just who is River Song may have been the fans’ most debated question of the season.