Professor River Song is a companion of the Eleventh Incarnation of The Doctor
- a time traveller
supposedly from the 51st century who has a long and complicated past with The Doctor. However, The Doctor is yet to experience this relationship in his current timeline: 'We keep meeting in the wrong order', as she describes it. The Doctor first met River in his 10th incarnation on a Library Planet, where River sacrificed herself to save the thousands of people trapped in the Library, and since then as they've met more and more, The Doctor knows River more, and River knows The Doctor less.
Eventually, on an Asteroid called Demon's Run, River revealed that she was in fact the grown up daughter of The Doctor's companions, Amy
and Rory Pond. As a baby, originally named Melody Pond, she was kidnapped and brainwashed to kill The Doctor by a sinister cult of aliens known as The Silence. Conceived by her parents whilst on The Doctor's Timeship, the TARDIS
, River's DNA was infused with the power of the Time Vortex, giving her the regenerative qualities of The Doctor's people, the Time Lords. However, after the brainwashed Melody attempted to kill The Doctor in 1930's Berlin
, her mother Amy broke the effects of the brainwashing and convinced her to give all of her Regenerations in order to save The Doctor's Life. Melody was left in the 51st century to recover, where she took the name River Song and pursued a career as an Archaeologist, knowing she was destined to meet The Doctor and her parents again.