premise: Donna Noble travels with the Tenth Doctor.
hypothesis: Donna Noble . Portrayed by British actress and comedian Catherine Tate , she is a companion of the Tenth Doctor ( David Tennant ) . Originally appearing in the closing scene of the show 's 2006 series and as a special guest star in its following Christmas special , `` The Runaway Bride '' , Tate was not expected to reprise her role as Donna ; for series 3 ( 2007 ) , the Doctor travelled alongside medical student Martha Jones ( Freema Agyeman ) . Donna becomes an asset to the Doctor on his adventures and it is she who ultimately saves the universe in the series 4 finale , although tragically at the cost of the memories of her travels with the Doctor . In a contrast to the Tenth Doctor 's prior companions , who both harboured romantic feelings for him , Donna and the Doctor shared a strictly friendly relationship ; and she did not feel the need to prove herself to be allowed to travel with the Doctor , who refers to her as his `` best mate '' . Tenth Doctor . This incarnation 's companions include working class shop assistant Rose Tyler ( Billie Piper ) , medical student Martha Jones ( Freema Agyeman ) , and fiery temp worker Donna Noble ( Catherine Tate ) .