Which episode introduced the character of River Song?

The episode that introduced River Song was “Silence in the Library.”

First broadcast on 31 May 2008, the episode is the opening part of a two-part Doctor Who story completed by “Forest of the Dead.” It introduces River Song, played by Alex Kingston, as an archaeologist leading an expedition into a planet-sized library in the 51st century. She already knows the Tenth Doctor, although he has not yet met her in his personal timeline.

River’s introduction immediately creates one of the show’s most memorable mysteries. She carries a diary resembling the TARDIS and knows details about the Doctor that convince him they will eventually become close allies. Her whispered knowledge of the Doctor’s real name establishes that their relationship will unfold out of order.

A common mix-up is answering “Forest of the Dead,” because that episode concludes River’s first story and contains her apparent sacrifice. However, her first appearance is in “Silence in the Library,” written by Steven Moffat. The story also introduces the Vashta Nerada, shadow-like creatures that feed on living flesh.