The Ninth Doctor (Christopher Eccleston) regenerates into the Tenth Doctor (David Tennant) due to cellular damage caused by absorbing the energies of the time vortex at the climax of "The Parting of the Ways". In the untitled Children in Need mini-episode, the Doctor initially exhibits stable behaviour as he introduces his new form to Rose Tyler, showing particular interest in his appearance, but soon begins acting erratically and says that his regeneration has "gone wrong". He remains in a delirious or comatose state through most of the events of "The Christmas Invasion" until his regeneration is settled through absorbing the free radicals and tannin from some hot tea that had dripped onto a power source inside the TARDIS. He then saves the Earth from invasion by killing the leader of the alien Sycorax using a satsuma when he refuses to leave peacefully ("No second chances; I'm that sort of a man", he remarks). The Doctor's right hand is severed in the fight, although he regenerates a new one since his regeneration cycle was not fully completed.
The Tenth Doctor and Rose go on to meet Queen Victoria, saving her from a werewolf which resulted in not only the Doctor's knighting as "Sir Doctor of TARDIS" and banishment from the British Empire, but also resulted in the creation of the Torchwood Institute, an Earth organization dedicated to policing supernatural and alien affairs. After being reunited with his previous incarnation's companion, Sarah Jane Smith, the Doctor, Rose, and Mickey Smith end up in an alternate universe where the Cybermen originate on Earth. After stopping them, the Doctor leaves Mickey behind at his request. During the 2-part event of "The Impossible Planet" and "The Satan Pit", the Doctor encounters the Torchwood Archive as they unknowingly awakened the Beast, who spoke of Rose's death to her shock as the Doctor assures her that it was only words. But the Beast's prophecy comes true when the Doctor eventually returns to present-day Earth, his TARDIS is stolen by the Torchwood Institute, who are manipulated by the Cybermen into creating a link to their universe through the Void, which inadvertently frees the Dalek Empire from the Time War as well. The Torchwood Institute forms a truce with the Cybermen until the Daleks are returned to the Void, at the cost of Torchwood's main base, the scattering of the Cybermen through time and space, and Rose, believed dead to the general public, ending up at the alternate universe with her mother and Mickey.
Immediately after the loss of Rose, who was in love with him, the Doctor meets Donna Noble, a bride drawn to the TARDIS during her wedding by a poisonous energy administered to her by the Racnoss, a race of spider aliens that had been hibernating in the Earth's core and needed to use Donna as a key to release themselves. The Doctor cures Donna and murders the Racnoss Queen and her brood, planning to die with them until Donna saves him. The Doctor offers Donna companionship, which she refuses but she suggests the Doctor finds someone who can "stop [him] sometimes".
The Doctor later saves Martha Jones from an alien vampire pursued by the Judoon, and brings her on his various adventures, one of which has him meet the Face of Boe before his death and hearing his final worlds, "You Are Not Alone". In the far, far future, near the Heat death of the universe, after the TARDIS attempted to shake off Captain Jack Harkness, the Doctor learns the meaning behind the Face of Boe's final worlds when he and Martha inadvertently free the Master from a Chameleon Arch. The Master immediately steals the TARDIS and regenerates, returning to Earth in 2006 and becoming Prime Minister of the United Kingdom. When the Doctor and Martha return, hitching a ride on Jack's Vortex Manipulator, the Master captures the Doctor and Jack, using them to enslave the human race. Martha escapes capture and attempts to undermine the Master. With Martha's help, the Doctor eventually defeats the Master and reverses the year during which he nearly destroyed the universe. After the universe is safe, the Master is shot and dies in the Doctor's arms.
After Martha and Jack's departure in the aftermath of the Master's death, the Doctor finds himself on a replica of the Titanic and keeps it from crashing onto London on Christmas Day.
Donna Noble ReturnsEdit
Soon after, the Doctor once more crosses paths with Donna, who accepts his offer and becomes his companion. Throughout their journey to places like Pompeii and the Planet of the Ood, the Doctor and Donna receive cryptic mentions of their future as well as equally mysterious appearances of Rose. After helping Martha and UNIT against the Sontarans, the TARDIS takes the Doctor, Donna, and a confused Martha to the planet Messaline in an unknown time period where the armed human colonists' forceful extrapolation of the Doctor's DNA results in the birth of Jenny, whom the Doctor eventually accepts as his 'daughter,' only to be separated from her by the end of the episode after he believes her to have died, not knowing that she later returned to life once he left.
In the aftermath of "Turn Left" and as The Stolen Earth begins, the Doctor is warned by Rose through Donna that the world is endangered. Finding the Earth gone, the Doctor contacts the Shadow Proclamation, learning that Earth is the latest of a series of planets that vanished. With the aid of the Doctor's former companions Captain Jack, Martha and Sarah Jane Smith, the Doctor pinpoints their location and learns that Davros is the cause of the planets' disappearance. The Doctor arrives on Earth where Rose is waiting for him, however before they are reunited, the Doctor is mortally shot by a Dalek, evoking his regeneration which he manages to redirect towards his previously severed hand that is connected to the TARDIS, keeping the Doctor in the same form. That action led to Donna inadvertently causing a "human biological metacrisis" by touching the severed hand, causing her to manifest into a Time Lord/human hybrid version of the Tenth Doctor to be created as well as Donna's eventual transformation into the predicted Doctor Donna that led to the Daleks' defeat and the worlds returned to their rightful places. After saying goodbye to his former companions, the Doctor is forced to erase Donna's memories when the Time Lord energy within her starts to consume her and leaves her behind as he departs in the TARDIS by himself.
In the following special episode, "Music of the Spheres", his musical talent is revealed, when he has taken his mind off his loneliness by composing an "Ode to the Universe".
In "Planet of the Dead", after returning stranded humans from San Helios, the Doctor is told of a premonition: "Your song is ending, soon. It is returning, it is returning from the dark. And then Doctor... oh, but then...he will knock four times", an event which will precede his death. It starts to become a reality during "The End of Time", when the Master is resurrected and rewrites humanity's genetic makeup in his own image, as an unwitting part of a plan designed to enable the Time Lords to escape their fate at the end of the Time War. After the Master and the Time Lords are stopped and the planet is safe, the Doctor finds his companion, Wilfred Mott, locked in a containment chamber in danger of imminent fatal contamination. Wilf knocks four times to alert the Doctor to his situation. Though raging against the universe for his fate, the Doctor nevertheless enters the cubicle to save Wilf, causing himself to suffer a lethal dose of radiation poisoning. The Doctor spends the remainder of his life paying final visits to his past companions, his last being Rose Tyler in a time before she met his previous incarnation. Dying a slow, but painless, death, the Doctor sets a new course in the TARDIS before he begins to regenerate, his last words being "I don't want to go". With a massive release of golden energy that cripples the TARDIS, he regenerates into a new actor (NIk Kershaw) as The 10th's doctor clone