On the planet Shan Shen, the Doctor and Donna are touring the markets. Donna wanders off on her own and a fortune teller offers to tell her the future for free. Donna accepts as the fortune teller draws the curtains closed behind her. The fortune teller speaks of Donna meeting a remarkable man and asks when her life crossed with his. Donna explains she had a job as a temp and feels a small shock as she remembers. She’s unaware that something is crawling on the floor behind her. The fortune teller talks of what choices led Donna to the job, and Donna mentions how six months before her mother wanted her to take a permanent job as a secretary. The fortune teller pressures Donna to reveal the exact point when she choose, when she turned left to go to her temp job. The fortune teller wonders what would have happened if Donna had turned right, then tells Donna to turn right. The creature on the floor, a beetle-like creature, crawls up Donna’s back and Donna in the past… turns right.
In the new past, Donna meets with her friends for Christmas at the local bar, and celebrates her promotion. One co-worker, Alex, stars at Donna’s shoulder even though there’s nothing there. They’re interrupted when a man comes in to tell everyone there’s a Christmas star outside. They run outside to see a glowing white spaceship in the sky above London. It hovers over the city and fires blasts of energy, but Alex runs in horror from whatever she sees on Donna’s shoulder. As tanks open fire on the ship, Donna moves to the ship’s location where U.N.I.T. has found the Doctor’s corpse: he didn’t make it out of the Racross base in time and drowned before he could regenerate. Donna notices the Doctor’s sonic screwdriver on the ground but as she walks toward it, a blonde woman comes running up: Rose Tyler.
Rose asks Donna her name and then says she shouldn’t be there, but something is wrong. She is also fascinated by something on Donna’s shoulder but when Donna turns back ground, Rose has vanished.
Later, Donna is fired from her job because of budget cuts due to the devastation caused by the Racross ship. They’re interrupted when an explosion shakes the building. They look outside to see a storm cloud forming over the hospital. The hospital disappears as Donna’s co-workers watch on TV. Donna is more interested in packing and leaving, taking a raffle ticket with her. Another shockwave shakes the building and the hospital reappears with only one survivor, who talks about talking rhinos. Donna is at home watching the reports with Wilfred when the survivor, medical student Oliver Morganstern, talks about how Martha Jones sacrificed her life to keep him alive. Donna doesn’t believe that aliens are responsible but Wilfred worries that alien incidents have been on the increase. Morganstern mentions that a Sarah Jane Smith and her teenage friends used a MRI to defeat the aliens.
As Donna walks home, she sees a flickering light in an alleyway. Investigating, she finds Rose, who says she didn’t see anything. Rose is still trying to see something on Donna’s shoulder, irritating Donna. Rose suggests that Donna and her family should leave London the next Christmas, and the raffle ticket’s prize is a free trip. Rose still refuses to give her name and when she refuses, Donna walks away.
The next Christmas, Donna, Sylvia and Wilfred go on the raffle trip. Come Christmas morning, the maid brings in breakfast and notices something on Donna’s back. They’re interrupted when a news broadcast announces that a replica of the Titanic
is falling out of the sky onto Central London. They hear an enormous explosion and run outside to see a mushroom cloud over London. They realize that if Donna hadn’t won the raffle, they would have all died.
With the whole of southern England flooded with radiation, people are being relocated anywhere possible after other countries close their borders. Donna and her family are shipped to Leeds where they find themselves crammed into a flat with dozens of other refugees. Wilfred tries to look on the bright side of things, since America has promised financial aid. However, they get the news that America has reported 60 million deaths when people dissolved into fat, which is transported off the planet by alien spaceships. They are left to live in the kitchen while Sylvia remembers all of her dead friends and Donna promises to find work. However, when they start to get settled in, they hear gunshots outside. The ATMOS devices in the cars are emitting poison gas and one of the soldiers sees something on Donna’s back He tells her to turn around but can find nothing.
Donna sees the same flicker of energy and goes to investigate, and finds Rose waiting for her. She explains that the ATMOS devices are spreading poison gas all over the world, as part of a plan conceived by the Sontarans. Rose describes how a small group of fighters have boarded the ship in orbit, and looks up at it blows up, incinerating the gas. She says it was the Torchwood team: Gwen Cooper and Ianto Jones gave up their lives and Captain Jack Harkness was teleported to the Sontar homeworld.
Donna notices Rose is wearing the same clothes but Rose explains that none of it was supposed to happen and a “wonderful man,” the Doctor, was supposed to stop everything. She explains that Donna traveled with the Doctor and saved his life below the Thames, but without Donna there he died. Donna starts to remember and Rose warns that something worse is coming and they need the Doctor. Rose has been pulled across from her parallel universe because “the darkness” is coming and it threatens all universes. Donna insists she’s nothing but Rose insists she’s important. Donna refuses to go with Rose but the latter says that Donna will have to come with her… and will die. Rose then fades away.
The foreign refugees are moved to labour camps, and Wilfred warns that it’s happening again. Donna’s had no luck getting jobs and that night, she and Wilfred go out to look at the stars through his telescope. He looks up and sees something ominous: the stars are going out one by one. Donna turns to find Rose waiting for her, and says that she’s ready.
Rose takes Donna to a U.N.I.T. base where Captain Magambo shows them a recovered artifact: the TARDIS. Donna goes inside and finds it larger on the inside than the outside. The ship is powered down but their using its energy to rig up a device to show the creature on Donna’s back to her. Rose explains the creature feeds off of time by altering someone’s personal history. It’s in a state of temporal flux and no one can touch it, but reality is bending around Donna. An angry Donna demands to know what she can do and Rose explains they’ve used the TARDIS to track down the moment of intervention. They’re going to send Donna back to stop herself from turning right, and restore the timeline. However, Donna realizes that she is going to die no matter what.
Donna arrives at the designated time but half a mile away. With four minutes left, Donna runs for the intersection but realizes she won’t make it. With seconds remaining, Donna realizes she’s going to die no matter what and throws herself in front of a delivery van. The traffic backs up to the intersection, while Rose arrives to tell the dying Donna two words that she whispers in her ear. Donna has no choice but to turn left, and in the real world she wakes up as the creature falls off her back, dying. The fortune teller runs in terror, frightened by Donna’s strength of will.
The Doctor finds Donna and examines the dead creature, and explains that it’s used by the Trickster’s Brigade. Normally it only affects the person it’s attached to but somehow it created an entire parallel world from Donna’s choice. The Doctor notes that it’s been happening to Donna a lot, and that the level of coincidence that has brought them together is more then mere coincidence. Donna starts to remember what Rose said to her about how all worlds were affected and the darkness is coming. Donna describes Rose and repeats the two words she was told: “Bad Wolf.” The Doctor runs outside only to see the words “Bad Wolf” all over the marketplace. Even the TARDIS is covered with them. The Doctor and Donna enter the TARDIS as the Cloister Bell rings, and the Doctor warns that “Bad Wolf” means… the end of the universe.