The Doctor and Donna Noble visit Shan Shen planet with a Chinese influenced market place. Donna is persuaded to visit a fortune teller who asks what life would be like if she hadn't met The Doctor. A beetle like creature makes its move towards the Donna and eventually attaches itself to the companion. Donna is transported back in time.
Donna is persuaded to turn right instead of left therefore joining a different company and not H.C. Clements where she doesn't meet the Doctor. Later on there's an alien attack with one confirmed death, the Doctor. From that point on, we seem parts from previous episodes turn out different. The hospital that disappeared in Smith and Jones disappears as it had done before and when it reappears, there's only one survivor, whose life is saved by Martha Jones.
Other incidents include the space Titanic crashing on London and obliterating the city and killing millions. Donna, her mother and Grandad Mott are transported to Leeds where they are forced to share a house with other families. Eventually one of the families is taken away as they do not fit the British norm for a labour camp with echoes of the concentration camps of the Nazis during the Second World War.
Throughout the episode, Rose Tyler appears at different intervals and she eventually tells Donna what is going on. The sky is going dark, all the stars in the constellation of Orion disappear when Donna looks through her grandfathers telescope. Rose tells and shows Donna the giant creature on her back. Donna is teleported back in time to correct the timeline by turning left as she should have done. To make Donna turn right, the time travelling Donna commits suicide in front of a van forcing the other Donna to turn right.
Everything returns to normal and Donna tells The Doctor that he met a previous companion who only said 'Bad Wolf', this alerts the Doctor who runs off and then sees posters everywhere saying Bad Wolf.