The Doctor meets River Song for the first time as he and Donna face the Vashta Nerada!
In the episode, the Doctor and his companion Donna Noble (Catherine Tate) land in the 51st century to visit the greatest library in the universe, encompassing an entire planet, but are baffled when they find it deserted. To the best he can determine, the library has been closed for 100 years so the arrival of a team of archaeologists led by River Song (Alex Kingston), comes as something of a surprise. They discover the Vashta Nerada, carnivorous creatures living in the shadows, are responsible. All they have is one warning—count the shadows.
This episode is also significant for introducing the character of Song into the mythology of Doctor Who; Song would go on to play an important role in future stories. A few days before the episode aired, the BBC announced that Moffat was to become head writer of the programme for the show’s fifth series in 2010, replacing then-current head writer Russell T Davies, who held the role since Doctor Who returned to television in 2005.