K-9 Mark IV TARDIS

K-9 Mark IV served as a replacement for the destroyed K-9 Mark III.

[edit] BiographyAfter Mark III sacrified himself to defeat the Krillitanes, the Doctor left a new model, Mark IV, for Sarah Jane Smith, just seconds after he departed in the TARDIS. K9 refers to himself as being rebuilt and improved, suggesting Mark IV possesses the same memory as Mark III. (DW: School Reunion) Though Sarah Jane didn't find this out right away, the Doctor had also placed some gifts in a panel in K-9's side: sonic lipstick and a wristwatch that could detect alien life. (Sarah Jane Smith website)

A year before Sarah Jane first met Maria Jackson, K-9 Mark IV left Sarah Jane in order to stabilize a dangerous artificial black hole in space which had been created in Switzerland. He could only speak with Sarah occasionally via a cabinet with a spacetime link when the orbit of the black hole intersected with that of Earth and did not know when he would complete the job. (SJA: Invasion of the Bane) However, at times he was capable of returning to Earth briefly, and did so on two later occasions in order to render vital assistance. In the first time, he fought Mr Smith in pitched battle to allow Sarah to upload the Armaggedon Virus, (SJA: The Lost Boy) And again in order to quickly upload the TARDIS base code to Mr Smith and assist the Doctor in returning The Earth to its correct location in space. (DW: "Journey's End")