The Doctor (David Tennant), shot to death by a Dalek, is regenerating in the TARDIS, but he transfers the regeneration energy into his severed hand in a jar—enough to heal him but not enough to change his physical appearance. Gwen (Eve Myles) and Ianto (Gareth David-Lloyd) of Torchwood hide from the Daleks in an impenetrable time-bubble, and Sarah Jane (Elisabeth Sladen) is saved from the Daleks by Rose’s (Billie Piper) mother Jackie (Camille Coduri) and ex-boyfriend Mickey (Noel Clarke).
The TARDIS is captured by the Daleks and taken to their flagship The Dalek Crucible. Sarah Jane, Mickey, and Jackie surrender in order to board the ship, the Supreme Dalek (voice of Nicholas Briggs) orders the TARDIS destroyed with Donna Noble (Catherine Tate) locked inside. Donna touches the severed hand, filled with regeneration energy, causing an identical clone of the Doctor to appear. The meta-crisis saves the TARDIS from destruction.
Davros (Julian Bleach), creator of the Daleks, explains that the planets the Daleks stole in the last episode, including Earth, form a reality bomb which will destroy all matter in every universe. To stop the bomb, Martha (Freema Agyeman) threatens to destroy Earth, while Sarah Jane, Mickey, Jack (John Barrowman), and Jackie threaten to destroy the Crucible. The Supreme Dalek transports both groups in front of Davros. Donna and the clone of the Doctor arrive and try to focus the bomb’s power into the Daleks. Davros blasts them with electricity.
This imbues Donna with Time-Lord knowledge from the clone, and she disables the bomb and the Daleks. The two Doctors help her relocate the missing planets, but the control panel is destroyed before Earth can be returned to its place. The clone Doctor destroys the Daleks and the Crucible. The original Doctor offers to save Davros, but he refuses the help. The companions flee into the TARDIS and, all together at the console, tow the Earth back into its original orbit, using the power of the spatio-temporal rift in Cardiff.
Sarah Jane returns home. Martha and Mickey leave with Jack. The Doctor returns Rose and Jackie to the parallel universe, sending his clone along with them. He is part human and can grow old with Rose. Donna’s human mind is being overwhelmed by the Time-Lord knowledge and begins to deteriorate. The Doctor wipes her memory to save her, telling her mother and grandfather that she will die if she ever remembers the Doctor. Then, he leaves, alone and lonely.
The episode is the culmination of all four reasons produced by Russell T. Davies. The music accompanying the restoration of Earth is the Song of Freedom of the Ood. Castell Coch in the UK is used as the German castle. The cloned Doctor was not CGI, but played by a musician named Colum Regan, who is a good match for David Tennant. There is a series of flashbacks showing all the people who died for the Doctor—sixteen of them. The episode was four years in creation—the same amount of time it took to create the Marvel Avengers movie series.
The episode was a spectacular finale, not as richly praised as the previous episode, but very popular. It was the most watched show in the UK that week. Donna’s end was appreciated for its sadness. Some called the episode chaotic and demented rather than dazzling, but readers of Radio Times called it the greatest finale of Doctor Who. The BBC considered ending the whole series at this point, but outgoing producer Russell T. Davies talked them out of it. There have been three Doctors since.