At a carnival on a Distant Planet with the Doctor (David Tennant), Donna (Catherine Tate) gets a free reading from a fortune-teller who has her recall meeting him. She recalls being at an intersection while looking for a new job. She wants to turn left for one job and her mother wants her to turn right. The fortune-teller makes her choose right instead. A large beetle attaches itself to her back and she passes out.
Her decision creates an alternate reality in which she never met the Doctor. He drowns in the flood while killing the children of the Racnoss Spider Queen. As a result, Sarah Jane Smith dies, Martha Jones dies, Captain Jack Harkness, Gwen Cooper, and Ianto Jones of Torchwood all die. The spaceship Titanic crashes into Buckingham Palace, setting off the nuclear fuel in the ship and killing millions. The UK is placed under Martial law and all non-British citizens go to internment camps.
Rose Tyler mysteriously appears and gives Donna advice which saves her and her family from destruction, but they are forcibly displaced. After noticing that the stars are disappearing, Donna decides to come with Rose. Rose can travel between universes now because reality is collapsing. Only the Doctor can stop that.
Donna is transported back in time to minutes before arriving at the intersection. She walks in front of a passing truck and is hit. The resulting traffic jam causes her former self to turn left instead. As Donna lays dying on the ground, Rose whispers a message for the Doctor into her ear: Bad Wolf. Donna wakes up back with the fortune-teller, who is terrified. The Doctor and Donna find Bad Wolf everywhere. The Doctor rushes into the TARDIS and hears the cloister bell ringing. The universe is about to end.
Since the Doctor was prominent and Donna almost not there in the companion-lite episode Midnight, Donna is prominent and the Doctor almost not there in this Doctor-lite episode, so they could both work on the big two-parter coming up. The story was influenced by the movie Sliding Doors. Rose describes Donna as the most important woman in creation. Writer Russell T. Davies was surprised at the death toll without the Doctor. The Giant beetle on Donna’s back compares with the giant spider on Sarah Jane’s back in Planet of the Spiders. A lot of stock footage was used.
Billie Piper admitted she had forgotten how to play Rose and had to watch some episodes. Some reviewers didn’t care for her performance, but I didn’t see a problem. David Tennant only worked for a day. Behind all this was the Trickster, who fed off changes in the timeline. In the Fires of Pompeii, a soothsayer tells Donna she has something on her back. It received mostly positive reviews, called extraordinary and powerful. It served to set up the big double-bill climax to the season.