A 9-year-old "Doctor Who" fan got a special Christmas gift from the exiting Doctor: a letter reassuring him that regenerating is good for the show.

With lead actor Peter Capaldi leaving the hit sci-fi series, he handed over the reins to the 13th Doctor (Jodie Whittaker) in the Christmas special. While the transition wasn't met with enthusiasm in the case of at least one young fan, the 9-year-old boy in question was lucky enough to get some kind words from Capaldi. The actor sent him a handwritten letter, which the boy's father, Brian McGilloway, posted on Twitter Wednesday.

Capaldi was the young fan's favorite Doctor, so he had been "dreading his regeneration," according to his dad. In the letter, the actor acknowledged that "regenerating is not completely good fun" but promised his young fan that it is always good for the show. He wrote:

Dear David,

I hope you are having a very merry Xmas. Xmas is always good fun. Well not always. Not every single second. Regenerating is not completely good fun. And it usually happens about Xmas-time. But you know what? Even though it can be a little bit icky (like really bad flu) it has always, always turned out good for Dr. Who. The new Doctor always becomes your favourite and the one that goes ... well, he never really goes, he is always there, somewhere in time and space, and if you think about him hard enough you'll see him, and he'll see you.

It's like the Doctor says, "Everything ends, and it's always sad. But everything begins again, and that's always happy. Be happy."

So, have a brilliant Xmas, a happy new year and a wonderful life -- I'm sure you will.

All the best,

Peter Capadli

Doctor ?

Capaldi himself became the Doctor during a Christmas special back in 2013, and not only did the show survive that regeneration, it had done so numerous times before then. Whittaker's debut is just another new beginning -- but the first for a female Doctor.

"Doctor Who" returns in fall 2018.