Discuss Turn Left here.

While visiting a market on the planet of Shan Shen (may be written as Xiang Xien) with the Doctor, Donna Noble is offered a free fortune reading. The fortune-teller presses Donna to reveal her past and focuses on a point where she was driving to her temporary job at H. C. Clements, despite her mother's desire that she take a permanent job nearby. As a large beetle-like creature climbs onto Donna's back, the teller convinces Donna to change her mind in the past, taking a right at the road junction per her mother's wishes instead of a left to H. C. Clements.

The narrative turns to the alternative history created by Donna's choice, far bleaker than the course of events established in previous episodes. The Doctor dies permanently during the Racnoss's attack on London ("The Runaway Bride"), killed by the water pressure before he could regenerate, because Donna was not there to convince him to leave. Royal Hope Hospital is taken to the moon and returned ("Smith and Jones"), but only one person, Martha's fellow medical student Oliver Morgenstern, survives. Martha Jones, Sarah Jane Smith, and her friends are among the dead (Sarah Jane apparently having foiled Florence Finnegan's plan). The Titanic crashes into the centre of London, wiping out the city and irradiating most of southern England ("Voyage of the Damned"). In the United States, 60 Million people are turned into creatures made of fat, known as Adipose ("Partners in Crime"). The Sontarans' attempt to turn Earth into a breeding world ("The Poison Sky") is stopped by Jack Harkness and his remaining Torchwood team of Gwen Cooper and Ianto Jones. Gwen and Ianto are killed, however, and Jack is transported to Sontar.

Throughout all these events, Rose Tyler keeps appearing before Donna. Aware of the events to come, she steers Donna away from mortal danger,e.g. she tells her to go to a hotel at Christmas, (forseeing the Titanic attack, as the parallel world is ahead of the normal world) but refuses 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 tells Rose 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 a 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 half a mile away and with only four minutes to spare. Finding herself short of the mark on the road leading from the right of the critical intersection, Donna remembers what Rose said about her death and throws herself in front of a removal van. Traffic backs up to the intersection and the past Donna turns left, unwilling to wait for it to clear. 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 in terror, frightened by Donna's strength of will. When the Doctor nonchalantly walks in, he explains that the beetle normally affects only the person it attaches to (the universe merely compensates), but in Donna's case it created a parallel world. The Doctor is curious about the other alternate realities that seem to form around Donna ("Forest of the Dead"). He ponders the coincidences surrounding Donna and himself, as if something is binding them together. When Donna insists that she is nothing special, the Doctor tells her that she is brilliant, which triggers her fading memories of Rose. She tells him about Rose's warning that "the darkness is coming" and that it is affecting all worlds. At his insistence, Donna tells him the words Rose said; "Bad Wolf". The Doctor instantly recognizes this as a message from Rose, and horrified, runs outside to find that the words "Bad Wolf" are everywhere, even on the TARDIS. Inside the Cloister Bell is ringing and the TARDIS's interior is glowing red. When Donna asks about the meaning of "Bad Wolf", the Doctor replies, "It's the end of the universe."

Fab acting between Catherine and Billie.