Doctor Who is a British TV show that's been around for a very long time. It went off air for 15 years or so and re-started in 2005. It's about a Time Lord- he's an alien, though he looks human- and his TARDIS, a time machine. He's simply called "the Doctor". He goes on all sorts of adventures and saves the world.
Whenever the character dies, he "regenerates". Every cell in his body is made new. Same memories, same past, same person... different man. It's confusing.
But any show which goes on for 60 years needs to change actors a few times, eh?