The long-running British science fiction television series Doctor Who has featured a large number of characters throughout its history. The show stars its protagonist, the Doctor, an alien who travels in time and space in a time machine known as the TARDIS. When the Doctor dies, they can regenerate, which allows them to survive death, though changes their appearance and personality in the process. The Doctor often travels alongside a number of travelling companions, who often act as major supporting characters and co-protagonists on their travels. The Doctor and their companions often encounter a number of antagonists, as well as a number of villainous alien races and creatures, who they must often stop from achieving villainous agendas.