Captain Harry Carruthers was a friend and employee of King Edward VII in 1902. Carruthers was described as "extremely handsome", and a "fine shot" with a gun after serving in the Boer Wars. However, he did not enjoy shooting, and had reservations about hunting deer. (PROSE: Revenge of the Judoon)
Carruthers was a close friend of Sherlock Holmes creator Arthur Conan Doyle. Doyle had been the Carruthers family's doctor before he became an author.
When Doyle was given a strange device by Professor Challoner he handed it to Carruthers so that he could then present it to the King. Carruthers took the device to Balmoral Castle, not knowing it was a plasma beacon which would allow the Judoon to remove the castle from its current position.
Carruthers assisted Martha Jones in discovering where Conan Doyle had found the mysterious object, and then assisted Martha and the Tenth Doctor in rescuing the King from the Judoon and Professor Challoner. (PROSE: Revenge of the Judoon)