Doctor: David Tennant (Tenth Doctor)
Companions: Catherine Tate (Donna Noble)
Billie Piper (Rose Tyler)
Bernard Cribbins – Wilfred Mott
Jacqueline King – Sylvia Noble
Joseph Long – Rocco Colasanto
Noma Dumezweni – Capt. Magambo
Ben Righton – Oliver Morgenstern
Chipo Chung – Fortune Teller
Natalie Walter – Alice Coltrane
Loraine Velez – Spanish Maid
Jason Mohammad – BBC Newsreader
Lachele Carl – Trinity Wells
Bhasker Patel – Jival Chowdry
Directed by Graeme Harper
Written by Russell T Davies
Script editor Brian Minchin
Produced by Susie Liggat
Executive producer(s) Russell T Davies
Incidental music composer Murray Gold
Production code 4.11
Series Series 4
Length 50 minutes
Originally broadcast 21 June 2008
The episode opens with the Doctor and Donna exploring a marketplace on the planet Shan Shen. Donna is approached by a fortune teller who convinces her to get a free reading. The fortune teller helps Donna recall the event that led to her meeting the Doctor. Donna remembers when she was driving along with her mother, Sylvia, through London to get a new job; at an intersection, Donna wanted to turn left to get to the security firm H.C. Clements for a well-paid temp position, while Sylvia wanted her to turn right as to take a job as a secretary at a friend's business. Donna ultimately turned left. The fortune teller gives Donna the chance to choose again and encourages her to turn right this time. As she makes this new choice, a large beetle attaches itself to her back.
Donna's decision creates an alternate universe with major changes in history. Donna initially seems happy with her secretary position, her only concern being the claims of her co-workers about seeing a beetle on her back. However, turning right meant that Donna never met the Doctor and could not convince him to flee after killing the Racnoss children in "The Runaway Bride". As a result, the Doctor drowns in the resulting flood, and is unable to intervene in several other events that drastically affect the Earth. Changes include the deaths of Sarah Jane Smith and Martha Jones in the events of "Smith and Jones", the death of sixty million people in the United States from the breeding of the Adipose in "Partners in Crime", and the loss of the Torchwood team while defeating the Sontaran invasion that occurred during "The Poison Sky". The Doctor's absence also allows the space-going Titanic to crash into Buckingham Palace during "Voyage of the Damned", killing millions in the subsequent destruction of London. England rapidly descends into a dystopia, with many Londoners displaced and forced into assigned cramped living quarters under martial law, with the government soon transferring all non-British citizens into internment camps.
Rose Tyler mysteriously appears to Donna to warn her of these events, but never reveals her identity to Donna. Rose's advice saves Donna and her family from the destruction of London. Sometime later, Rose appears again and tells her that she needs Donna to come with her, but that doing so will result in Donna dying. Donna refuses, and Rose tells her that she will be back in three weeks. Three weeks later, Donna and Wilfred are using his telescope when they realise that the stars are disappearing from the sky. Rose appears and Donna is now convinced to go with her. Rose takes her to UNIT and explains that the stars are going out in every universe, and the fabric of reality is collapsing, allowing Rose to travel between universes. Rose implores to Donna that the Doctor is the only one who can stop it.
Donna is taken to a dying TARDIS, which UNIT has managed to use as the core of a crude time machine. Rose uses energy from the TARDIS to show Donna the beetle on her back, but tells her they do not know what it is. She insists that Donna travel back to the day she made the fateful turn and choose to turn left again. Donna is transported back minutes before she would make that turn, but too far away to contact her past self in time. She makes the decision to walk out in front of a passing truck and let it hit her, creating a traffic jam that causes her past self to turn left out of impatience. As Donna lies on the ground dying, Rose whispers a message into her ear for the Doctor. The alternate universe disappears, and Donna finds herself back on Shan Shen. The beetle falls off of Donna's back, and the fortune teller runs away after uttering some terrified words about what Donna will become.
The Doctor hears Donna scream and turns up in the booth. After examining the beetle, the Doctor comments on how it was odd for an alternate universe to form around her, explaining that the beetle worked for The Trickster with the job of changing people's lives in various ways. His remarks cause Donna to recall Rose and her message, which she delivers to the Doctor. Rose told her two words, "Bad Wolf". A panicked Doctor, realising who Donna must have met, runs out of the room to find the words "Bad Wolf" written everywhere once again. He rushes into the TARDIS only to hear the cloister bell, and announces that the universe is in danger.