The Tenth Doctor
"I don't want to go."
Despite how alien he is, the Doctor still feels love, and this was especially true of his tenth incarnation, as his first companion, Rose, captured his hearts more than most, even though he was never able to admitted this to her.
The Doctor didn't have time to mourn when they were tragically separated, as he was straightaway swept up into a new mystery when feisty bride Donna materialized inside the TARDIS. Together they saved the world from the Empress of the Racnoss, however when he invited her to join him on his travels, she declined. She explained that sometimes he needs to be told when to stop, so she urged him to find somebody else to share his adventures with.
He did just that when he stumbled across the Royal Hope Hospital, which was harbouring a deadly Plasmavore. The inter-planetary thugs, the Judoon, transported the hospital to the moon so they could seek out the criminal, and it was here that the Doctor met Martha, who was training to become a medical student. Martha saved the Doctor's life and in return he offered her one trip in the TARDIS to say thanks. The one trip soon grew into many more, and together they encountered the likes of William Shakespeare, the Weeping Angels and the Daleks. Their travelling together eventually came to an end when, after defeating the Master, Martha decided to stay home to be with her family.
Just moments after their farewell, the Doctor was thrust into yet another adventure when a luxury space cruiseliner called the Titanic crashed through the TARDIS walls. Onboard, he fought against the sinister Host alongside a waitress called Astrid Peth, who would've accompanied the Doctor on his travels permanently had she not given her life to foil Max Capricorn's plans to destroy the Earth.
The Doctor was left to travel the stars on his own. Whilst infiltrating the mysterious Adipose Industries company, he was reunited with none other than Donna Noble, who was also undercover posing as a health and safety inspector on a mission to track down her old friend.
On the Chino planet of ShanShen, Donna was attacked by a creature of the Trickster's Brigade, creating a whole new world in which she never met the Doctor, who had perished whilst drowning the Racnoss and her children. Donna joined forces with Rose and together they set time back on its right course, just in time to help the Doctor and the rest of the Children of Time stop Davros and his Dalek fleet from destroying all of reality.
The Doctor started to regenerate after being shot down by a Dalek during his reunion with Rose, but used his old hand in a jar to heal himself before the process was fully complete. Later, Donna touched the jar, resulting in the first ever Time Lord/human meta-crisis, and subsequently a half-human Doctor was born, while Donna's brain absorbed all of his intelligence.
With the human Doctor safely on the parallel universe with Rose, the Doctor was forced to wipe Donna's mind, and as as a result she lost all of her memories of her time in the TARDIS and he returned her home to continue the ordinary human life she had before him. Alone again, he journeys to London in 1851. Here, he defeats the Cybermen's latest scheme with the help of a man who claims to be the Doctor, however it soon transpires that he is actually a human called Jackson Lake, whose mind had been confused by a Cyberman infostamp.
Back on present day Earth, the Doctor introduces himself to criminal Lady Christina de Souza on a bus seconds before it's transported to San Helios, a planet ravaged and devoured by stringray-like creatures known as the Swarm. When they're back to safety, fellow passenger Carmen warns him 'it is returning through the dark' and that 'he will knock four times'.
A quick visit to Mars becomes the Doctor's toughest challenge yet when he is forced to battle with the knowledge that its first ever human colony, Bowie Base One, is soon to be destroyed by a deadly virus in the water, killing all of the crew and their captain, Adelaide Brooke. The Doctor - realising that he no longer has to obide by the rules of time - decides to save Adelaide, Yuri and Mia, but Adelaide later commits suicide, reversing all of the changes the self-proclaimed 'Time Lord Victorious' had made to the original timeline.
The Tenth Doctor died during a final showdown with the Master and the Time Lords. When Wilf knocked four times on a chamber which was about to be flooded with lethal doses of radiation, the Doctor entered it in his place and thus his latest regeneration began. In the time he had left, he visited each of his companions as his final 'reward'.