While enjoying time on the Chinese-style planet of Shan Shen, Donna Noble is offered a free fortune reading. The fortune-teller presses Donna to reveal her past and focuses on a point in her past on modern-day Earth where she was driving to her first day to her temporary job at H. C. Clements, despite her mother's wishes to take up a permanent job nearby. As a large beetle climbs onto Donna's back, the teller convinces Donna to change her mind in the past, taking a right at the intersection per her mother's wishes instead of a left.
The narrative turns to the alternative history created by Donna's choice, far bleaker than the established course of events. The Doctor dies during the Racnoss' attack on London, Donna having not been there to make him leave. Royal Hope Hospital is taken to the moon and returned, but only one person survives. Martha Jones and Sarah Jane Smith are among the dead. The Titanic crashes into the centre of London, wiping out the city and irradiating most of southern England. In the United States, 60 million people are turned into creatures made of fat. The Sontarans attempt to turn Earth into a breeding world, which is stopped by Jack Harkness' Torchwood team. However, they are all killed and Jack is stranded on Sontar.
Throughout all these events, Rose Tyler keeps appearing before Donna, steering her away from mortal danger but refusing to give her name. After the latest tragedy, Rose urges Donna to come with her, even though she will die. Donna initially refuses, but three weeks later, as she and her grandfather talk about recent events, the stars begin disappearing throughout the sky. Donna realizes Rose is there, and tells her that she is ready.
Rose escorts Donna to a UNIT base where the dying TARDIS is being used to help power a makeshift time machine. Rose uses the system to show Donna the beetle that crawled onto her back during the fortune-telling. It is in temporal flux and cannot be removed, but Rose explains that Donna herself is also point of flux. In order to set things right, they prepare to send her back in time to stop herself from going right. Donna agrees to go, but when she asks if she will get to live this time, Rose remains silent. Donna is sent back in time, but ends up a half-mile away and with only four minutes to spare. Falling just short of the mark, she realises what Rose meant about her death and throws herself in front of a removal van. Traffic backs up; the past Donna hears the sound of the disturbance, and turns left instead of waiting. As the future Donna lies on the ground, Rose leans over and whispers two words to pass on to the Doctor.
Back on Shan Shen, the beetle falls off of Donna's back and the fortune teller flees, scared of Donna's willpower. The Doctor finds Donna and the beetle. He explains that it normally affects only the person it attaches to (the universe merely "compensates"), but in Donna's case created a parallel world. The Doctor is curious about the other alternate realities that seem to form around Donna, and this triggers her fading memories of Rose. She tells him about Rose's warning that "the darkness is coming" and that it is affecting all realities. At his insistence, Donna tells him the words Rose said; "Bad Wolf". The Doctor runs outside to find that the words "Bad Wolf" are all about them, even on the TARDIS. Inside the Cloister Bell is ringing and the TARDIS interior is coloured red. The Doctor realizes this means one thing: "the end of the universe."
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