The Doctor is a renegade Time Lord from the planet Gallifrey, who became a hero who fought evil and injustice across the universe, and in violation of the Time Lords' non-interference policy, was exiled. Technically he explored only with the intention of experiencing the wonders of the universe and having fun, often traveling with a companion, but he frequently found trouble wherever he went.
Alone among the Time Lords, he survived the Last Great Time War with the Daleks, though they returned shortly before his tenth regeneration. Throughout his life, he had a particular association and affinity with the planet Earth and its Humans. Thanks to his eternal heroics, he is a well known and celebrated individual throughout time and space.
He and his current companion Donna Noble visited the leisure planet Midnight, where Donna chose to stay at the leisure palace whilst the Doctor took a four-hour shuttle bus ride to the Sapphire Waterfall. The bus was subsequently attacked by an unknown creature which possessed one of the passengers, Sky Silvestry and eventually the Doctor, with the aim of inciting mass hysteria among the other passengers. Before the creature could trick the passengers into throwing out the Doctor, Sky was dragged out into the X-Tonic sun by the bus' hostess, who realized that Sky was stealing the Doctor's voice, and the creature departed, its plans defeated.
Next, the Doctor and Donna visited the Chinese influenced planet of Shan Shen. When Donna went to get her fortune told, a member of the Trickster's Brigade created an alternate universe around her. In this alternate universe she met Rose Tyler, who told her two words to tell the Doctor. After Donna corrects the universe, she is able to tell the Doctor the two words: Bad Wolf. Donna asks what it means and the Doctor replies with "The end of the Universe".
He rushed back to Earth to check that everything was fine, but as soon as they went back to the TARDIS, the whole planet was stolen. Finding no clue as to where the Earth was, he went to the Shadow Proclamation and learned that 27 planets including Earth had been stolen. With a bit of help from Harriet Jones, Jack Harkness, Sarah Jane Smith, Luke Smith, Martha Jones, Mr Smith and Rose Tyler, as well as Donna's mother and grandfather, he was able to be contacted and managed to lock on to the location of the Earth. After a quick chat with h is friends and Davros (who had been saved by Dalek Caan from the Last Great Time War), he landed on Earth.
While talking to Donna, Donna pointed out that Rose was behind him. He turned around and the two began to run towards each other. Then a Dalek emerged from the shadows and fired at him. It was only a glancing blow but was enough to mortally wound him. He had to be taken back into the TARDIS where he began to regenerate. Using his nearby severed hand, he used the regeneration to heal himself, but not to regenerate into a new form.
As a result of this, a new Doctor was born from his severed hand and Donna was accidentally made part Time Lord, becoming the DoctorDonna of the Ood prophecy. DoctorDonna disabled the planet powered Reality Bomb to stop Davros (creator of the Daleks) from destroying the entire universe and then the Daleks and the three Doctor's sent the planets back to their original time and space.
However, the Doctor was horrified by the actions of the new Doctor, who wiped out the last remaining members of the Dalek race, knowing they had the numbers to take the Universe by force. As the Doctor's companions fled to safety in the TARDIS, the Doctor offered to save Davros and Caan. Davros refused, hatefully calling the Doctor, "The Destroyer of Worlds".
The Children of Time then used the TARDIS to tow Earth back to its original point in time and space, there he said his goodbyes to his companions. He left the new Doctor with Rose Tyler in hopes that she could help change his mannerisms, just as she had helped him. (Mun views the Doctor and Rose's relationship as strictly platonic, not romantic.)
Upon returning to our universe, he was forced to remove Donna's memories of him, as they would eventually kill her; her Human mind not being able to withstand his Time Lord memories. He returned Donna to her mother and grandfather, and told them that they would have to make sure she never remembered anything about him, or their adventures together.