Time travel gets lonely. As such, every incarnation of the Doctor has chosen – or inadvertently taken aboard – a companion. Usually, the companion is a young woman, but this rule isn't set in stone. Together, the Doctor and his companion travel through time and space, exploring the universe and all of its crazy, endless wonders. From a writing standpoint, the companions are meant to help the viewer get into the show easier. The Doctor explains everything in detail to his companion and vicariously to the audience.