After the departure of Rose Tyler from the new series, the task of her replacement on board the TARDIS was never going to be an easy job, particularly given The Doctor’s own residual feelings for Rose. However, during a subsequent adventure to Earth, The Doctor met a woman who more than met the requirements to face the perils he tackled on a regular basis; Martha Jones, a young medical student working in the Royal Hope Hospital.

Smith and Jones

 When The Doctor and Martha originally met, The Doctor had infiltrated the hospital posing as a patient to try and find a blood-sucking alien known as a Plasmavore ("Smith and Jones"). Although Martha initially thought him to be just a normal, if demented, human being, she soon came to trust him, helping him by keeping the Judoon - alien bounty hunters tracking the Plasmavore - occupied while The Doctor ensured that the Plasmavore would be scanned by the Judoon as alien; the Plasmavore could ‘mimic’ a human appearance by drinking blood, but The Doctor tricked her into drinking his blood and causing her to register as alien to the Judoon’s scanners. Although The Doctor nearly died as a result of the blood loss, Martha managed to get him breathing again by giving him CPR, and, in gratitude, The Doctor offered to take her on a brief trip in the TARDIS, the ‘brief’ trip soon extending into a second journey, and then a third, before The Doctor finally invited Martha to become a full-time companion on his travels.

 Like all companions, Martha’s time with The Doctor was never going to be easy, but in her case it was always going to be difficult for personal reasons rather than just because of the usual chaos The Doctor would encounter. From the beginning, Martha knew that she was essentially a ‘rebound’ companion - The Doctor still not having quite recovered from the abrupt loss of Rose Tyler at the conclusion of "Army of Ghosts/Doomsday" - feeling understandably frustrated when The Doctor took her back to New Earth, a location that he had once visited with Rose. Despite this, however, The Doctor became fond of Martha in her own right, going to great lengths to save her when she was abducted during their visit to New New York ("Gridlock") and praising her intellect when she acquired a DNA sample from the recently-mutated Richard Lazarus ("The Lazarus Experiment"), although he still only ever saw her as a friend even after she began to fall in love with him.

 Over the course of her adventures, Martha soon established herself as a more independent companion than Rose; while Rose regularly stayed in contact with her mother, Martha was almost grateful for the opportunity to get away from her family, having been forced to organise all their conflicting schedules for them. Despite the strained relationship between them, however, Martha clearly cared for her family, regretting the tension that developed between her and her mother as a result of the ‘warnings’ her mother had been given about The Doctor from the mysterious ‘Harold Saxon’. Her independence also demonstrated itself in her ability to handle problems without The Doctor; during a trip to New York, while The Doctor was occupied with the Cult of Skaro, Martha single-handedly came up with a plan to defeat the Cult’s genetically-engineered pig-slaves by channelling the electricity from the lightning conductor atop the Empire State Building into the lift that the slaves were using to reach them, thus electrocuting the pigs before they could even try to attack Martha and her current allies ("Daleks in Manhattan/Evolution of the Daleks").

The Shakespeare Code

 Also unlike Rose, who had only definitively defied The Doctor when dealing with matters relating to her family - most notable when she prevented her father’s death - Martha regularly acted alone simply because she felt it was the right thing to do, risking her life by returning to an apparently artificially-generated forest to help two hunters fighting a ferocious beast despite The Doctor’s concerns that the reality in question could be about to cease existing ("Wooden Heart"). Although she was, like all companions, prone to getting captured, such as when she was temporarily held hostage by the insane Professor Challenor who sought to eliminate almost all of Earth’s major cities with Temporal Reversion devices, Martha never gave up; in the above situation, even when bound hand and foot she took action against her captor, knocking him into the device after The Doctor had destroyed his console ("Revenge of the Judoon").

 However, Martha’s true moment of glory occurred with the return of The Doctor’s old enemy The Master, the only other surviving Time Lord, who had hidden himself at the end of the universe disguised as the human Professor Yana to escape the Time War that had annihilated the rest of their race. Having stolen the TARDIS, The Master had cannibalised it to create a Paradox Machine, using this machine to draw the Toclafane - future humans who had mutilated themselves to survive before agreeing to serve The Master - into the present, where they decimated Earth without erasing themselves, The Master also using his new laser screwdriver to age The Doctor nine hundred years, turning him into a weakened old man who could only watch as the world he loved was destroyed. With nobody else to help her, Martha escaped using the vortex generator of The Doctor’s old companion Jack Harkness, subsequently travelling the world to tell everyone about The Doctor and how he had saved them for so many centuries, simultaneously passing on an instruction that was fulfilled when she finally returned to the Valiant a year later. As The Master activated the countdown to trigger the missile launch that would begin his war against the universe, the true purpose of Martha’s travels was revealed; as the countdown hit zero, every human being on Earth began to chant the word ‘Doctor’, based on the instructions Martha had given them… and, with every mind on Earth linked via the telepathic field created by the Archangel Network, The Doctor was able to tap into the minds of all humanity and use that telepathic energy to restore himself to full health, subsequently disarming The Master and giving Jack Harkness the opportunity to disable the Paradox Machine.

Human Nature/The Family of Blood

 Despite this victory, Martha subsequently told The Doctor that she had decided to leave him; officially, this was because her family needed her to cope with the trauma of the year-that-never-was, but, as she later admitted to him, the real reason was that she didn’t want to end up like a friend of hers who stayed in a student house with the same person for five years because she was in love with him when he never noticed her as anything more than a friend. Understanding her reasons, The Doctor accepted her decision and, after one last embrace, he and Martha parted ways, although Martha did leave The Doctor her mobile phone in case she ever wanted to get in contact with him again.

 Following her qualification as a doctor, Martha joined UNIT - aided thanks to a recommendation letter The Doctor had left for her - although she briefly transferred to Torchwood Three to help them investigate recent mysterious deaths and the subsequent resurrection of team medic Owen Harper after he was shot and restored to a state of ‘living death’ (His body being physically dead while still capable of independent thought and action). She also met The Doctor and new companion Donna Noble when she called upon The Doctor’s aid to investigate the new ATMOS car navigation system - really a prelude to a Sontaran attempt to invade Earth - during which a clone of her was created to help the Sontarans infiltrate UNIT; although The Doctor saw through the clone almost instantly, he allowed it to remain active so that he could feed the Sontarans disinformation, the clone later dying after Martha was freed (The Sontarans had been forced to keep her alive so that her brainwaves could be broadcast to the clone in order to make it more convincing).