Melody Pond

Professor River Song was a mysterious archaeologist and convicted murderer who shared a close relationship with the Doctor. Significantly, she was the only companion who knew the Doctor's true name that he told her at some point in his future for reasons as of yet undisclosed. She would often greet him by saying "Hello, sweetie" whether it be on a message or in person.

Although his first meeting with her ended in her death, they had many adventures — from the Doctor's perspective — afterward. Like Melanie Bush before her, she was an example of the rare companion whose timeline was not synchronous with that of the Doctor.

Early life
River Song was an archaeologist already highly familiar with the Doctor when they first meet (from his perspective) in the 51st century. She was, in fact, a future companion of his who travelled with him off and on. According to her, he came to trust her implicitly, though at the start of their relationship (from his perspective) he was generally suspicious of her and almost hostile to her at times. Prior to her encounter with the Weeping Angels, she was imprisoned in the Storm Cage Holding Facility for the murder of someone she described as, "A very good man. Best man I've ever known." (DW: The Time of Angels / Flesh and Stone)

Opening of the Pandorica
In the 52nd century, an imprisoned River Song received a phone call from Winston Churchill, who was calling from 1941, attempting to call the Doctor. She escaped from prison after kissing a new guard whilst wearing hallucinogenic lipstick, and stole an old van Gogh painting, which foretold a deadly prophecy of the TARDIS exploding, from Liz 10, who confronted her about her intentions. River then made a deal at a black market for a Time Agent vortex manipulator that she used to travel back in time to the oldest planet in the universe. There, she wrote a message on the cliff face to the Doctor, telling him to go to the Roman era of Earth. Once there, she disguised herself as Cleopatra and used her hallucinogenic to fool the Romans. She explained the situation to the Doctor once he arrived and showed him the painting of the TARDIS exploding. They then proceeded to Stonehenge where they search for the Pandorica. They located it, but then discovered that Stonehenge was transmitting a signal, bringing in Daleks, Cybermen, Sontarans, and many other alien races. The Doctor managed to convince the assembled aliens to back away for a time, and sent to bring the TARDIS to him. However, the TARDIS brought River to Amy's house, on the 26th June, 2010. Looking around, River discovered burn marks scorched into the garden by alien craft and the front door broken distached from the house, and found a book on Romans and another on Pandora's Box in Amy's room, the book on Romans containing the exact likenesses of the Roman troops in 102 AD. Returning to the TARDIS to inform the Doctor, River then found herself trapped as the TARDIS as it came under control of an unknown external force and started to explode. (DW: The Pandorica Opens)

Inside the TARDIS, River is stuck in a time loop of her last few moments until the Doctor (using her vortex manipulator) is able to retrieve her and bring her back to 1996, where she mercilessly kills the Dalek responsible for shooting the Doctor. After the Doctor restores the universe to life (erasing himself in the process), River leaves her (now blank) diary with Amy Pond on her wedding day in 2010 to trigger her memories of the Doctor, thus reviving him. When she and the Doctor part ways, she apologises for something in the future and answers "Yes" to a question she interprets both as "Will you marry me?" and "Are you married?" before teleporting off with the vortex manipulator. Before she leaves however, she tells the Doctor he will know who she really is very soon, but apologizes, warning him that that's when everything changes. 

Crash of the Byzantium
Sometime after these events she was presumeably imprisoned once again, however in an earlier century. She remained so until she was put under Father Octavian's custody. If she were to carry out the mission assigned to her she would earn her pardon. (DW: The Pandorica Opens)

Doctor River Song was a guest on the Byzantium ship in the 51st century. She was working for a military organisation at the time in exchange for the pardon of a murder. She infiltrated the lower levels to find the Home Box, into which she carved the message "Hello Sweetie" in the Old High Gallifreyan language to attract the Doctor's attention. About 12,000 years later the Doctor found the Home Box in a museum. The Doctor used the cube to see who wrote the message. Seeing her being pursued by guards and giving him the co-ordinates of her location, he went to rescue her. She then jettisoned herself from an air lock, where the waiting Doctor and Amy Pond picked her up in the TARDIS. River then showed exceptional TARDIS flying skills landing it next to the Byzantium, which had crashed into the Maze of the Dead. Along with the Bishop and his Clerics, River accompanied the Doctor and Amy into the maze. She made it out of the maze alive along with Amy and the Doctor, to be imprisoned once more, but she thought she may have done enough to earn a pardon. She then mentioned to the Doctor about their meeting at the Pandorica. (DW: Flesh and Stone)

Death
River's expedition to the Library was financed by Felman Lux and was chartered to discover what disaster had happened there, years previously, that had caused 4022 people to "die" or disappear. When her team ran into trouble during the expedition, River summoned the Doctor by sending him a message via psychic paper. The version of the Doctor who turned up had not yet met her. Though bewildered, he agreed to help River and her team. River and the Doctor appeared to be very close, even considering the strong bonds the Doctor forms with his companions. She kept a worn and battered diary, the cover looking very much like the TARDIS doors, keeping a journal of her travels with the Doctor. There was a suggestion that in the Doctor's personal future he would carry a similar journal, as she commented about needing to "compare" diaries. She carried an upgraded version of the sonic screwdriver which "her" version of the Doctor had given her.

As the situation in the Library deteriorated, River realised she must prove to the Doctor that she was, or will become in his case, someone he trusted completely and with profuse apologies, she whispers his true name into his ear. The Doctor was stunned by this information, and River succeeded in winning his trustRiver physically died when she chose to sacrifice herself in order to stop the Doctor from sacrificing himself in order to save Donna and the other 4022 people trapped in the Library's computer. She pleaded with him to not try and change history, not to change one moment of what is to come for them. After saving all the trapped people, the Doctor realized his future self must have given her his sonic screwdriver for a reason. Inside he found a data chip, which contained River's Data Ghost. The Doctor saved her by uploading her into the virtual world contained in The Library's data core. CAL, the library's data core, controlled by a human girl wired into its mainframe, also managed to save the data ghosts of all of River's dead archaeological team, so she would have company in the virtual world. River would go on to take care of two virtual children in the virutal world, reading out her diary of her life with the Doctor and how "everybody lives" when the Doctor comes to call, her story ending with her telling the sleeping children sweet dreams. (DW: Silence in the Library/Forest of the Dead) The Doctor would later remember her death when Davros reminded him of the people who had died in his name. (DW: Journey's End)

Other information

 * River Song seemed to take shorter trips with the Doctor, living a more or less normal life between trips, rather than having one extended trip with the Doctor and never seeing him again after parting ways.
 * River Song is a skilled TARDIS pilot (with very different methods from the Doctor). She has mentioned that she "learned from the best" but implied that her teacher was not the Doctor. (DW: The Time of Angels) Later, however, she revealed that the Doctor had indeed taught her. (DW: The Pandorica Opens) Although, by her first statement, she could have meant that she did not learn how to pilot the TARDIS from the Eleventh Doctor to be precise, but from another of his (later) incarnations.
 * Song appears to have knowledge of Time Lord anatomy, including knowledge of regeneration. She indicates that destruction of both of the Doctor's hearts is a circumstance after which regeneration is not possible.
 * River Song has the ability to write messages in High Gallifreyan.
 * River possessed pictures of all the Doctor's incarnations, though logicaly one must assume she does not know what order the doctor's incarnations were experienced due to her reaction during a meeting with the Doctor during his Eleventh Incarnation.
 * River is sentenced to prison for murdering a man described as "a hero of many" and whom she calls "A very good man. Best man I've ever known". Octavian felt that the murder victim's identity is something that the Doctor "really [doesn't] want to know."
 * Before his death, Octavian tells the Doctor that "You think you know [River], but you don't. You don't understand who or what she is.

River Song's timeline

 * Meets the Doctor for the first time.
 * Has an adventure with the Doctor at the Bone Meadows. (mentioned DW: The Time of Angels)
 * Has a picnic with the Doctor at Asgard. (mentioned DW: Silence in the Library)
 * Escapes from her Storm Cage in the 52nd Century. Obtains a Vortex manipulator, and writes the oldest writen message in history on a cliff face of Planet One. The message, of course found by the Doctor, reads Hello Sweetie, Theta Sigma (the Doctor's nick name) and coordinates for Britain in 102 AD. River then meets the Eleventh Doctor in 102 AD when the Pandorica opens. (DW: The Pandorica Opens)
 * Is trapped in a time loop in the TARDIS as it explodes. (DW: The Pandorica Opens/The Big Bang)
 * River is rescued from the TARDIS and brought back to 1996, where she mercilessly kills the Stone Dalek, who shot the Doctor. After the Doctor restores the universe to life (erasing himself in the process), River leaves her (now blank) diary with Amy Pond on her wedding day in 2010 to trigger her memories of the Doctor, thus reviving him. Afterwards, the Doctor returns it to her and it is filled again, but he says he didn't peek ahead. (DW: The Big Bang)
 * Parts ways with the Doctor, apologising for an event in the future, and replying "Yes" to a question she interprets both as "Will you marry me?" and "Are you married?" before teleporting off with the vortex manipulator. Before she leaves she tells the Doctor he will see her again very soon and will finally know who she is then, but apologizes as that's when everything will change. (DW: The Big Bang)
 * Kills "a hero to many", "the best man [she's] ever known" and is imprisoned in or before the 51st century. (mentioned DW: Flesh and Stone)
 * Father Octavian releases her from Storm Cage. (DW: Flesh and Stone)
 * Four days later, she breaches the Byzantium and is rescued by the Eleventh Doctor. (DW: The Time of Angels)
 * River Song and the Eleventh Doctor investigate the crash landing of the Byzantium. (DW: The Time of Angels)
 * Travels to the End of the Universe with the Doctor. (mentioned DW: Silence in the Library)
 * The Doctor, with a new haircut and a suit, visits River Song and takes her to see the Singing Towers of Darillium. He gives her his Sonic Screwdriver. (mentioned DW: Forest of the Dead)
 * Tries to send 'her Doctor' a message via the Psychic paper, asking him to come to the Library. (DW: Silence in the Library)
 * The Tenth Doctor, whilst travelling with Donna Noble, comes to the Library in the 51st Century at her request, where they face an infestation of Vashta Nerada within the Library. This is the Doctor's first meeting with River Song in his personal timeline.
 * Proves to the Tenth Doctor how important she will be to him by whispering his true name in his ear. (DW: Silence in the Library)
 * Sacrifices herself to save the Tenth Doctor and the Library. (DW: Forest of the Dead)
 * The Tenth Doctor beams River Song from a neural relay in her sonic screwdriver into the Library's Data Core. (DW: Forest of the Dead)
 * River Song's brainwaves live on in the virtual world inside the Data Core, containing CAL and Doctor Moon. She is reunited in this virtual world with her crew mates, Proper Dave, Other Dave, Anita and Miss Evangelista, whose brainwaves must have also been added to the Data Core. (DW: Forest of the Dead)

''Some of the above events that have only been mentioned, may not have occured exactly where they have been placed here. For example, when River "travels to the end of the universe" with the Doctor, it may have been in her earlier or later life.''

Behind the scenes

 * It is unknown precisely where in time and space River Song originally came from. As the Doctor has left previous companions in different parts of the universe from where he originally found them, it is possible that River was not originally from the 51st century at all.
 * In a recent interview by Digital Spy, Steven Moffat made a passing comment to the effect that River Song might turn out to be very different from people's presumptions as to who she really is.
 * In Doctor Who Confidential, Stephan Moffat stated that we find out who River Song really is in series 6.