Continuing from the end of "The Stolen Earth", the Doctor (David Tennant) is regenerating inside the TARDIS. Once his body is healed, he halts the transformation by transferring the remaining energy into his severed hand. The TARDIS is captured by the Daleks and transported to the Dalek Crucible. Powerless, the Doctor and his previous companions Jack Harkness (John Barrowman) and Rose Tyler (Billie Piper) leave the TARDIS, but Donna Noble (Catherine Tate) is unwittingly locked in. The Daleks send the TARDIS to be destroyed; in the process, Donna collapses by the Doctor's severed hand and touches it. The stored energy flows into her then back into the hand, forming a new Doctor, who saves the TARDIS from destruction. Concurrently, Torchwood employees Gwen Cooper (Eve Myles) and Ianto Jones (Gareth David-Lloyd) find safety in an impenetrable time bubble; Sarah Jane Smith (Elisabeth Sladen) is saved from a Dalek extermination by Rose's ex-boyfriend Mickey Smith (Noel Clarke) and mother Jackie Tyler (Camille Coduri) and surrender themselves to get aboard the Crucible; and Martha Jones teleports to a Schloss sixty miles from Nuremburg which houses a UNIT station.
Aboard the Crucible, the Doctor and Rose are taken to Davros (Julian Bleach), creator of the Daleks. Davros explains that the twenty-seven stolen planets form a compression field for energy which can cancel the electrical energy of atoms. The resulting "reality bomb" has the potential to destroy all matter in every universe; reality itself would be destroyed. After the device is tested, the Daleks receive two transmissions: Sarah, Mickey, Jack, and Jackie threaten to use an explosive crystal to destroy the Crucible, and Martha threatens to use the Osterhagen Key — a last resort device which would destroy Earth. The Daleks teleport all five to Davros' chamber; Davros uses their actions to challenge the Doctor:
"The man who abhors violence [...] but this is the truth: you take ordinary people and fashion them into weapons. [...] How many have died in your name? The Doctor, the man who keeps on running, never looking back because he dare not, out of shame. This is my final victory, Doctor. I have shown you yourself."
Davros prepares to detonate the reality bomb seconds before the TARDIS materalises in the room. The other Doctor and Donna try to attack Davros but are quickly immobilised. Just before the bomb detonates, Donna becomes imbued with Time Lord knowledge as a side effect of the creation of the other Doctor. She uses a control panel to stop the detonation and disable Davros' guards. The two Doctors and Donna return the stolen planets to their original locations; Davros asks the precognitive Dalek Caan why he did not foresee this. Caan explains that he realised the Daleks' evil and conspired to destroy the entire race.
Before Earth can be relocated, the control panel is destroyed by a Supreme Dalek. As the Doctor prepares the TARDIS, the other Doctor fulfills Dalek Caan's foretelling, destroying the Daleks and triggering the Crucible's destruction. The Doctor offers to save Davros; he refuses, labelling the Doctor as "the Destroyer of Worlds". The companions flee into the TARDIS before the Crucible is destroyed and — aided by Sarah's computer Mr Smith, her robotic dog K-9, and the spatio-temporal rift in Cardiff — "tow" the Earth back into its original orbit.
In the dénouement of the episode, the Doctor parts ways with his companions: Sarah returns home to her son Luke; Martha and Mickey leave with Jack; and the Doctor returns Rose and Jackie to the alternative universe they were trapped in in "Doomsday". The Doctor forces the other Doctor to stay in the parallel universe as punishment for committing genocide and to requite Rose's love. After departing, Donna becomes overwhelmed by the Time Lord knowledge. The Doctor is forced to erase her memories of him, and explains to her mother Sylvia (Jacqueline King) and grandfather Wilfred Mott (Bernard Cribbins) that she will die if she remembers him. As the Doctor leaves, Wilfred promises to remember the Doctor on his granddaughter's behalf. The episode concludes with the Doctor in the TARDIS, left alone with his thoughts.
Doctor David Tennant (Tenth Doctor)
Companions Catherine Tate (Donna Noble)
Billie Piper (Rose Tyler)
Freema Agyeman (Martha Jones)
John Barrowman (Jack Harkness)
Elisabeth Sladen (Sarah Jane Smith)
Noel Clarke (Mickey Smith)
Camille Coduri – Jackie Tyler
Adjoa Andoh – Francine Jones
Gareth David-Lloyd – Ianto Jones
Eve Myles – Gwen Cooper
Thomas Knight – Luke Smith
Bernard Cribbins – Wilfred Mott
Jacqueline King – Sylvia Noble
Julian Bleach – Davros
Valda Aviks – German Woman
Shobu Kapoor – Scared Woman
Elizabeth Tan – Anna Zhou
Michael Price – Liberian Man
Barney Edwards, Nick Pegg, David Hankinson, Anthony Spargo – Dalek Operators
Nicholas Briggs – Dalek Voice
John Leeson – Voice of K-9
Alexander Armstrong – Mr Smith
Writer Russell T. Davies
Director Graeme Harper
Script editor Lindsey Alford
Producer Phil Collinson
Executive producer(s) Russell T. Davies
Production code 4.13
Series Series 4
Length 2nd of 2-part story, 65 minutes
Originally broadcast 5 July 2008