The original BBC program Doctor Who began airing in 1963, and continued into 1989, making it the longest running sci-fi program, and indeed one of the longest of any genre in TV history. An unsuccessful attempt was made by Fox to revive it in 1996. BBC tried again in 2005, and this attempt proved successful.
The program follows the adventures of a Timelord, one of a race of humanoid alien beings from the planet Galifrey. His name has never been revealed--he is known only as "The Doctor". He travels through time and space in a "Tardis", which stands for "Time And Relative Dimensions In Space". It is much bigger inside than outside.
In his travels, he encounters various monsters such as the Daleks, the Cybermen, and a renegade Timelord known only as The Master.
Several different actors have starred as the Doctor over the years. This is explained in the storyline as "regeneration". When the Doctor faces death, he cheats it by literally becoming a new man. The supporting cast also changes periodically.