 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".