The Doctor is a time traveller who is having adventures on a daily basis with terrestrian followers. His time machine is called the DARTIS (which is an acronym for "Distances Are Reduced! Temporally and In Space"). To his archenemies belongs the ancient race called the Dalooks, two-eyed metal creatures that only know curiosity and want to explore the entire known universe without permission by the central hegemonial power!

(Oh, you're asking why The Doctor is allowed to explore the universe? Don't worry: The Doctor fills out all forms for legal exploration and adventuring during his 20-year-stay on Bureaucratia. S01E002-S02E014)