"Doctor Who" is a classic science-fiction series with a cult following. The Doctor is called a "Time Lord," a time-traveling scientist from a far-off planet who travels through time and space in a shop known by the acronym TARDIS. A TARDIS is a machine that is larger on the inside than the outside and is supposed to change its appearance based on its surroundings. The Doctor is also able to evolve his biology, so he appears as many different people throughout the series. The Doctor loves Earth, so he makes many trips here to save the planet, or to enlist earthlings to help him with tasks in the galaxy.