Only 10 actors have played time traveler Doctor Who during the 46-year history of the world's longest-running science fiction series.

Soon it will be 11.

David Tennant, the current Doctor, appears in three more episodes of the BBC America series, starting with Saturday's "Doctor Who: The Waters of Mars." Then Matt Smith will take over the role.

Scottish actor Tennant will leave behind the role of a mysterious traveler who zips across time, space and universes in a craft called the Tardis, which looks like a British telephone booth from the outside but is way different inside.

Changing lead actors is part of series lore. The idea started when William Hartnell, the first Doctor, wanted to leave the series. It was decided that Time Lords, the race of time-traveler aliens that Doctor Who is part of, could regenerate into a whole new form.

"It's very exciting, but it's also very sad," Tennant says. "It's thrilling to be handing over the show in such good health, actually. So it's a whole mixture of emotions, actually, and probably until they actually transmit, I won't quite know how it feels."

The series was a hit in Britain long before U.S. public television stations started showing past episodes, most notable for their low-budget special effects. Off the air for almost a decade, the series was brought back by the BBC in 2005 with a much bigger special-effects budget and a ninth Doctor, played by Christopher Eccleston.

Tennant took over a year later.

Landing the role of the Doctor was beyond Tennant's acting expectations. A fan of "Doctor Who," all he wanted was a chance to work on one episode. The 37-year-old actor credits "Doctor Who" for making him want to become an actor.

He was 17 when he started his professional career and has worked mostly in British television, including the much-heralded "Viva Blackpool," and films such as "Harry Potter and the Goblet of Fire."

Over the years, each actor has brought his spin to playing the Doctor.

The ninth and 10th Doctors, written by Russell T Davies, creator and writer of the new "Who" series, were completely different: Eccleston's Who was quite serious, while Tennant brought a boyish playfulness to the role.

Tennant always liked how Doctor Who could be a bungler but at the end of the day could save an entire universe with a ball of string.

The decision to leave the role was hard for Tennant because of the global fan base, especially among "the geeks and nerds," a group that Tennant feels very much a part of. He opted to leave now rather than wait until a time when others might suggest he should go.

Because the Doctor's form can change, Tennant could return in the future.

"I'll wait for the correct opportunity," Tennant says. "I've got the costume hanging up in my wardrobe. And as long as I can keep my waistline and still fit into the trousers, never say never."