Martha Jones

Martha Jones, a medical student, encountered the Doctor at Royal Hope Hospital in London, which was transported to the Moon by the Judoon, alien police in search of a criminal.  After traveling with the Doctor and sharing many adventures with him, she returned to Earth and became a doctor.  She joined UNIT (the Unified Intelligence Taskforce) as a medical officer and worked at Torchwood in Cardiff, Wales with Captain Jack Harkness and his team.  Later on she transferred to the UNIT branch in New York, employing the top secret teleportation experiment Project Indigo using Sontaran technology.