The Doctor is an alien. He is a Time Lord from the planet Gallifrey. He travels in his spaceship called the TARDIS. It allows him to go anywhere in time and space. "TARDIS" stands for Time And Relative Dimension In Space. The Doctor usually takes other people with him, who are usually called "companions" or "assistants". The Doctor and his companions travel through space and time, have a lot of adventures, and often save many people.