Martha Jones is introduced in the third series (2007) of "Doctor Who", first appearing in the episode "Smith and Jones". When the hospital she works at is teleported to the Moon, medical student Martha helps save the day alongside an alien time traveller known only as The Doctor (David Tennant). To thank her for her help, The Doctor invites her to join him for a supposed single trip in his time machine the TARDIS, but later accepts her as his full-time "companion", admitting that she was "never just a passenger", and he even gives her the key to the TARDIS in the episode, "42". Martha becomes frustrated because The Doctor is oblivious to her feelings for him, and she expresses concern that she is simply a rebound after The Doctor's painful loss of his previous companion, Rose Tyler (Billie Piper). When the amnesiac Doctor falls in love in the two-part story "Human Nature" and "The Family of Blood", a pained Martha claims "You had to go and fall in love with a human... and it wasn't me". In the series finale, "The Sound of Drums" and "Last of the Time Lords", in which The Doctor's nemesis The Master (John Simm) takes over planet Earth, capturing both The Doctor and fellow companion Captain Jack Harkness (John Barrowman), Martha manages to escape by teleporting away but is left alone to save the world. On the run from The Master, she spends a year travelling the world in a plan which restores the incapacitated Doctor and reverses time, undoing The Master's actions. Able to remember the events during The Master's reign, Martha then leaves the TARDIS of her own accord, telling The Doctor that she cannot waste her life pining for someone when the relationship cannot happen, but promises that she will see him again. Martha, as voiced by Freema Agyeman, also appears in the 2007 animated serial "The Infinite Quest", which aired in twelve weekly segments during the run of the 2007 series.