"I'm Doctor Martha Jones. Who the hell are you?"
Martha Jones - a 23 year old medical student - meets the Doctor when the hospital she's training at is transported to the moon by the Judoon, and back on Earth, the Doctor invites her inside the TARDIS for a quick 'thank you' trip. She steps aboard, initially under the impression that he fancies her, however she soon realises that their relationship is purely platonic, and that he simply requires her company as he struggles to come to terms with losing Rose.
The Doctor appears to be instantly impressed by Martha's bravery and compassion and, after somewhat extending the 'one trip' rule, he presents her with the 'frequent fliers' privileges, giving her her very own TARDIS key as well as her mobile phone the 'Superphone' upgrade.
Throughout their travels,
she has stuck by his side during many life-threatening situations, and it's obvious that she would do anything in her power to protect him. After spending a whole year travelling the world sharing stories of the Doctor as part of a plan to restore him and ultimately defeat the Master, she decides to bid farewell to the TARDIS in order to care for her family, who have been left traumatised by the events they witnessed onboard the Valiant. It's not permanent, however, as she leaves her mobile phone and tells him that when it rings he is to come running back to pick her up.
Unbeknownst to Martha, the Doctor has recommended her to UNIT, who phone her up out of the blue and offer her a job as a medical officer. It's when UNIT are busy investigating the new ATMOS systems that Martha contacts her old friend, who arrives with Donna to help fight off the Sontarans. Together they foil their plans to turn Earth into a cloning planet, and, whilst saying goodbye in the TARDIS, its doors suddenly shut and they are taken to Messaline, where Martha is kidnapped by the Hath, a usually peaceful race of fish-like creatures who are at war with the cloned humans on the planet.
Martha grows close to one of the Hath in particular, Peck, who saves her from quicksand and sacrifices himself in order to rescue her. After ending the conflict between the two species, the Doctor takes her back home to her fiancé, Tom Milligan, and she doesn't hear from him again until Earth is stolen by the Daleks. Harriet Jones, former Prime Minister, contacts her through a subwave network, which brings together the Doctor and his companions so they can join forces to save the world. Using Project Indigo, a teleporting system salvaged by the Sontarans, she is taken to a top-secret pod in Germany. With the Osterhagen key in tow, she delivers an ultimatum to the Daleks - the key will detonate a series of strategically placed nuclear warheads beneath the Earth's crust if they don't agree to leave.
After the Daleks are defeated, Martha helps to fly the Earth back home in the TARDIS before leaving with Captain Jack and Mickey, seemingly to join Torchwood alongside Jack, Gwen and Ianto. However, the next time the Doctor sees her, she is fighting off extra-terrestrial beings on Earth as a freelancer alongside her new husband, Mickey.