 In 2005 , Russell T Davies had envisioned using a previous companion to explore the role and eventual fate of the Doctor 's companions . Sladen was convinced to appear in the second series episode " School Reunion " . Meanwhile , CBBC proposed a spin @-@ off to Davies about a teenage Doctor , which he denied in favour of a spin @-@ off revolving around Sarah Jane . While surprised , Sladen agreed to sign on the project . However , the rights for K @-@ 9 to appear in the series were not secured . To explain the non @-@ appearance of K @-@ 9 , the production team gave him a cameo fixing a black hole inadvertently created in Switzerland . 
