As the last of his kind, the alien Time-Lord simply known as the Doctor takes it upon himself to defend humanity against invading threats. He does this by traveling back and forth through time in a machine called the TARDIS (which looks like a blue telephone booth) to key moments where invading aliens interfere. He is not in this fight alone however, various people that the Doctor comes across, known as Companions, help him to defeat the invaders and get the time stream back to normal. The Doctor himself can go through physical changes called "regeneration" in order to save himself when he is near death, a neat plot device that lets new actors assume the title role every few years. Matt Smith has become the eleventh Doctor, replacing David Tennant in this long-running sci-fi series.