River Song is a fictional character created by Steven Moffat and played primarily by Alex Kingston in the British science-fiction series Doctor Who. River Song was introduced to the series as an experienced future companion of series protagonist the Doctor, an alien Time Lord who travels through time in his TARDIS. Because River Song is a time traveller herself, her adventures with the Doctor occur out-of-synchronisation; their first meeting (from the audience's perspective) is his first and her last. In later appearances, River is a companion of the Doctor in his eleventh incarnation, portrayed by Matt Smith. River Song was created by Doctor Who writer Steven Moffat for the show's fourth series in 2008, under the tenure of executive producer Russell T Davies. When Moffat took over Davies' duties as executive producer, he began expanding on the character's background, depicting adventures earlier in River's timeline, upgrading Alex Kingston from a guest star to a recurring actor in the series. Other actresses have subsequently portrayed younger versions of the character, from infant to child, to young adult.