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 .
To thank her for her help , the Doctor invites her to join him for one trip in his time machine the TARDIS , but he 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 .