Donna's World

Donna's World was, according to the Tenth Doctor, a "great big parallel world" — although parallel worlds were normally sealed off, Donna Noble had such a world "created around [her]" by the Time Beetle, one of the Trickster's Brigade. This was unusual, as most times when the Brigade changed time, the universe just compensated around it. In this world, Donna turned right instead of left while deciding which of two different job interviews to attend. As a result, the Tenth Doctor never met Donna and died underneath the Thames on Christmas Eve 2007. This meant that he also never met Martha Jones. These events resulted in devastating effects for Earth. Using the surface technology of the Doctor's TARDIS, Donna travelled back in time from 2009 to make her younger self turn left again. According to both the Doctor and Donna, this world subsequently ceased to exist. (TV: Turn Left)

History
While on the planet Shan Shen, Donna was unknowingly fitted with the Time Beetle, one of the Trickster's Brigade. A fortune teller selected a point in her life where making a seemingly random choice had enormous ramifications.

On Monday 25 June 2007, Donna had to make a choice between two jobs. The moment she chose between the jobs was at a junction on Little Sutton Street. Her agency had offered her a contract with H.C. Clements, which while temporary, was posh and in the centre of London, which Donna enjoyed. Her mother, Sylvia Noble, had got her a job opportunity to become the personal secretary of Jival Chowdry, owner of a photocopying business. Although this would be a permanent job, Donna found it less interesting.

While driving and in discussing the dilemma, Sylvia pressured her into turning right towards Griffin's Parade to stop by Jival Chowdry for a chat. While in the regular world Donna ignored her mother and turned left towards the Chiswick High Road, here she turned right, resulting in her taking the PA job. (TV: Turn Left)

On Christmas Eve 2007, the Tenth Doctor tracked down the Empress of the Racnoss to the Torchwood base underneath the Thames Flood Barrier and blew it up, causing the Thames to be drained into the abandoned Torchwood base. (TV: The Runaway Bride) Although he succeeded in defeating and drowning the Racnoss and its offspring, the Doctor drowned with them. For some reason, he didn't regenerate. It was speculated that he drowned too fast to regenerate. His body was retrieved by UNIT soldiers and his TARDIS was salvaged from under the Thames, later being sent to a UNIT base in Leeds. Donna stumbled across Rose Tyler, who arrived on the scene too late to do anything. (TV: Turn Left)

In 2008, the Judoon H2O scooped the Royal Hope Hospital to the Moon to find a Plasmavore criminal and her Slab accomplices. Although they found and killed the alien criminals, the Plasmavore had already armed the MRI. (TV: Smith and Jones) Sarah Jane Smith, Luke Smith, Maria Jackson, and Clyde Langer, who had been in the hospital when it was transported, disarmed the MRI, but they died, along with the rest of the hospital, when their oxygen ran out. The only survivor was Oliver Morgenstern, who was passed the last oxygen tank by medical student Martha Jones, who gave up her life to save him.

Rose visited Donna and advised her to persuade her family to spend Christmas some place other than London. On Christmas morning, a replica of the Titanic crash-landed onto Buckingham Palace. It destroyed the Greater London Area and flooded the whole of Southern England with radiation. Donna's family, who did decide not to stay in London for the holidays, having won a luxury getaway in a raffle, watched the events on television and were able to see the mushroom cloud of the explosion from a distance.

Britain went under martial law in order to deal with the crisis caused by the nuclear explosion. Many citizens fled to France, until France closed its borders. The United States promised fifty billion pounds in financial help for Great Britain but this plan was abandoned when in 2009, sixty million Americans were fully converted into Adipose children. (TV: Turn Left)

The Sontarans activated the ATMOS devices in the cars to convert Earth into a breeding ground. (TV: The Sontaran Stratagem / The Poison Sky) The Torchwood Three team, Captain Jack Harkness, Gwen Cooper and Ianto Jones, worked to stop the scheme. The team destroyed the Sontaran flagship and successfully reverted the atmosphere to its previous state, but Gwen and Ianto died in the attempt; Jack, being immortal, survived and was captured by the Sontarans and transported to Sontar as a prisoner. Britain was largely unaffected due to its poverty and hence its lack of petrol. Rose once more appeared to Donna, telling her that Donna knew the Doctor and travelled with him "in a different world", and that Donna was "meant to be there" under the Thames — the Doctor needed someone to stop him and save his life, and that was Donna. Rose asked Donna to come with her. Donna refused, but Rose told Donna that she will agree to come along in three weeks time and that she was going to die. Rose subsequently disappeared.

Roughly three weeks after the Sontaran invasion, the British government made a new law ("England for the English") forcing all non-British citizens in Britain to go to "labour camps". Donna's grandfather Wilfred lamented that labour camps was "what they called them last time", and "it's happening again." (TV: Turn Left)

In the Doctor's world, as a result of Davros' reality bomb, (TV: Journey's End) also referred to as "the darkness", (TV: Turn Left) using the rift at the heart of the Medusa Cascade to blast the bomb's Z-Neutrino wavelength into "every dimension", (TV: Journey's End) "every single universe" was in danger of being destroyed. (TV: Turn Left) Dimensions started to collapse, destroying everything; not just in the Doctor's world or the world Rose was in, but "the whole of reality". Even the Void was dead. (TV: Journey's End) Rose was "pulled across from a different universe" because of this. (TV: Turn Left)

With UNIT's help, Rose scraped off the surface technology of the TARDIS and planned to use a Lodestone to bounce chronon energy into the centre of a circle of mirrors, controlling the destination to send Donna back to Monday 25 June 2007.

UNIT and Rose sent Donna back to the point where Donna made her decision. According to Rose, Donna had to make her past self turn left instead of right at the point where she decided whether to follow her mother's advice or her own instincts on Little Sutton Street. Donna arrived in 2007, but too far away from her past self to directly influence her; instead, she orchestrated an accident by stepping out in front of a lorry, which blocked traffic and forced the earlier Donna to turn left. Just before Donna's death, Rose appeared and whispered a message for her to give the Doctor. Donna woke up on Shan Shen, and the Time Beetle was forced off of her back. The fortune teller ran away, questioning what Donna will be. Both the Doctor and Donna believed that the world ceased to exist. Donna couldn't remember much of her world, "like when you try and think of a dream, and it just sort of goes". However, Donna remembered Rose telling her that the Doctor thought she was brilliant. Not knowing Rose's name, Donna told the Doctor about the blonde woman at the lorry who said to warn the Doctor about the end of the universe with two words: "Bad Wolf". (TV: Turn Left)

Aftermath
On the Crucible in the Doctor's world, Donna prevented the reality bomb from ever being detonated by closing all Z-Neutrino energy loops using an internalised synchronous back-feed reversal loop. The collapse of dimensions closed up again because of dimensional retroclosure. (TV: Journey's End)

Behind the scenes

 * David Tennant did not actually play the person lying on the stretcher in the scene where they pull the body from the Thames, although they used the same suit that the Doctor wore in TV: The Runaway Bride.
 * Even without the Doctor's intervention, the plan of the Cult of Skaro in 1930 Manhattan during the two part television story Daleks in Manhattan/Evolution of the Daleks somehow still failed.