The Unknown (audio story)

 was the first story of the audio anthology, The Diary of River Song: Series Two, produced by Big Finish Productions. It was written by Guy Adams and featured Alex Kingston as River Song, Sylvester McCoy as the Seventh Doctor and introduced Salome Haertel as Rachel Burrows.

Publisher's summary
A planetary anomaly. A scientific impossibility. A mystery to be solved.

Of course, River Song expects to be consulted. She expects her valuable knowledge and experience will help the crew of the Saturnius unlock the strange phenomenon that has appeared in Earth's solar system.

But what River doesn't expect is a stowaway. An infuriating little man, calling himself the Doctor.

Plot
Professor River Song arrives to question a stowaway, whom the crew of her ship have locked up in a cell. The man is the Seventh Doctor, but River doesn't seem to recognise him. On the ship's bridge, Captain Maddie Bower and pilot Ellen Byrne watch the stars disappear one by one from the space around them, while in the engine room, engineer Robert Murphy begins to hear strange voices, which he tries to ignore.

River explains to the Doctor that he's aboard a peculiar spaceship, the Saturnius, built to navigate without consequences through fluctuations in space-time. A new planet has suddenly appeared in the solar system, and its gravity is causing numerous problems in the space around it; River and the others have been sent to investigate. The Doctor is a stowaway they found ... but they can't remember when or where. The Doctor himself notices that their memory doesn't seem to be working properly, experiencing gaps and distractions (that's why River has no memory of him); he's also affected, but manages to maintain more stability due to his unique biology.

Captain Bower decides to turn back, but when Byrne sets the course the whole ship is upset, risking collapse. At the Doctor's suggestion, River convinces the captain to reset the previous course to the mystery planet, then returns to the prisoner and asks how he knew what to do. The Doctor explains that it's simple: everything was fine as long as they were going towards the planet, it started to go wrong when they turned to go back (simple logic). However, this means that the ship is not in open space, as everyone assumed.

In the engine room, Robert Murphy keeps hearing voices, which assume the voice of the ship's computer - and invite him to exterminate the rest of the crew.

On the bridge, River talks to the captain and Byrne, and together they note the many discrepancies in their memories. The Doctor contacts them after escaping from the cell and hacking into their internal communications, claiming to know what happened; the three women visit him in the cell, where he has once again locked himself away. The Doctor explains that what is happening is the result of a triple collision between three different objects, all halfway between the Time Vortex and physical space: the ship they are travelling on, the mysterious planet and his TARDIS.

In the engine room, Murphy tries to resist the voices urging him to murder, but when he calls the captain to tell him something is wrong he hears her insult him, which diminishes his internal resistance to the voices.

River, who understands what the Doctor is saying, helps him explain to the other two what's going on, and then tries to work out with him a way to resolve the situation. The Doctor suggests going to find his TARDIS: once it's at the controls, he can use it to distance his ship, turning back a few seconds to avoid impact. At that moment, Murphy's call for help comes through: a few seconds, but enough for the Doctor to realise that Murphy is near the engine room, at the shield engines protecting the ship from fluctuation - the centre of danger in the ship. Byrne is sent back to the bridge to deactivate the shields remotely, while River, the Doctor and the captain proceed to the engine room to retrieve the TARDIS.

The journey is made difficult by the fact that the further they go, the more reality warps and time seems to slow down, even creating doubles of themselves from different moments in their lives. To avoid this, the three must stay focused on the present, or at least try. Byrne arrives at the bridge and is about to deactivate the shields, when a crazed Murphy, now a man-machine hybrid, attacks and kills her.

The other three continue on their way, encountering a troop of Murphy's duplicates, who River kills, much to the Doctor's disappointment. They finally arrive at an area of the ship where the technologies of the TARDIS and the Saturnius appear to have merged. The Doctor attempts to approach the console, but the three are attacked again by Murphy's duplicates, who kill the Doctor when he tries to deal with them. The captain then decides to allow herself to be split and duplicated in turn, allowing River to get to the controls of the TARDIS. Even in her current state, River's memory is restored enough to operate the controls correctly when she comes into contact with the TARDIS console to disconnect the two ships, thus partially resolving the paradox.

River and the Doctor awaken in the Saturnius ' brig and the TARDIS respectively, and although they don't remember what happened, they have retained the consciousness necessary to avoid imminent impact, changing course at the last minute. River also convinces the captain and Ellen to deactivate their shields, making them realise how close they actually were to the planet that was their destination. The crew manages to arrange a safe landing instead of crashing to the ground.

As they go out to check, River and the captain notice that time seems to be moving forward and backward very quickly on the planet. When they come across an oak tree, River (who knows that such trees only grow on Earth) realises that they have landed on an Earth in an alternate future, where the world has been destroyed by an unknown catastrophe. On their way back, the two stumble upon what looks like a dead body, but is actually an android, named Rachel, who suddenly reactivates.

Cast

 * River Song - Alex Kingston
 * The Doctor - Sylvester McCoy
 * Maddie Bower - Anna Maxwell-Martin
 * Ellen Byrne - Gemma Saunders
 * Robert Murphy - Justin Avoth
 * Computer - Dan Starkey
 * Rachel - Salome Haertel

The Doctor

 * The Doctor stated that he was "overdue a nap", referring to him rarely needing sleep.
 * The Doctor says that one of his best friends is an archaeologist.

River Song

 * Even when her memories are interrupted by the numerous temporal anomalies, the name "the Doctor" rings a bell somehow.

Technology

 * According to the Doctor, he patented the sonic screwdriver.

Plants

 * According to River, during cultivation of Earth plant species on other worlds, the oak tree could never thrive.

Continuity

 * The Doctor recognises that River's device is a sonic screwdriver, (TV: The Husbands of River Song, Silence in the Library et al.) and claims to have invented this technology himself. (AUDIO: The Quantum Possibility Engine; TV: The Doctor Dances, A Christmas Carol)
 * The closer the crew gets to the Doctor's TARDIS, the more the walls of the two ships have combined. One crew member notices "the round things". (TV: The Day of the Doctor)
 * River Song is able to pilot the TARDIS with ease, even when her memory has been interrupted. (TV: Let's Kill Hitler, et al.)
 * The temporal anomalies are made worse by two time vessels colliding in the Time Vortex. One is the Doctor's TARDIS. (TV: Time Crash)
 * The TARDIS has fused with a spaceship before, similarly mixing up their architecture. (TV: Terminus)
 * River Song says that Oak trees are not found on any planet but Earth. (TV: The Android Invasion)
 * The future timeline of Earth has been altered. (TV: The Mysterious Planet, PROSE: The Book of the War, COMIC: Short-Change)