Douglas Henderson was a living weapon. He was created during the Last Great Time War and was the last of his type as such weapons were subsequently outlawed by the Shadow Proclamation. Douglas was unaware of his purpose and believed himself to be human.
Douglas lived on Earth. Throughout his life he kept seeing a blue box that was always followed by trouble. One day he found himself pursued by an alien. He came across the box and the Tenth Doctor who fended off the alien.
They entered the box and the Doctor told Douglas of his true nature. They were taken to a ship orbiting Earth. It was crewed by aliens who were at war with the race who had chased Douglas. They each wanted Douglas as a weapon.
Douglas activated himself, disabling both fleets. The Doctor revived him and he had enough power left to live out a normal human life. Douglas thanked the Doctor and never saw him or his blue box again. (COMIC: The Big, Blue Box)