Doctor Who fans around the world await 50th anniversary special

Doctor Who fans around the world should expect an "emotional wallop" at the end of the 50th anniversary episode, says the show's boss Steven Moffat.

"It's the most ambitious episode we've ever done," he said.

The episode, The Day of the Doctor, will be broadcast in more than 90 countries at the same time as it airs on BBC One on Saturday night.

The BBC says it is likely to be the largest simulcast of a TV drama in history.

"This event means it is a worldwide show not simply a British phenomenon," Moffat said.

The episode will also be screened in 3D in more than 1,500 cinemas across the world, including Australia, Russia, Brazil, Mexico, Ecuador, Spain, Sweden, Norway and Iceland.

The 50th anniversary adventure stars Matt Smith, David Tennant and John Hurt as different incarnations of the Doctor.

John Hurt's version of Doctor was first introduced at the end of the episode, The Name of the Doctor, broadcast in May.

Jenna Coleman plays companion Clara, while Billie Piper returns as Rose.

The episode will also feature the Daleks and the return of shape-shifting aliens the Zygons, who first appeared in 1975.

The story involves a mystery at London's National Gallery in the present day, as well as "a murderous plot" in Elizabethan England in 1562.

Guest stars include Jemma Redgrave as Kate Stewart and Joanna Page as Elizabeth I.

The Zygons, not seen since the mid-70s, make a return in the anniversary special

Steven Moffat, Doctor Who's lead writer and executive producer, admitted he was "nervous" about the special episode finally being seen around the world.

"I'm glad we don't do it every time, but it's very exciting to do it once," he told the BBC News website.

He added he hoped fans would be "very happy" with the 75-minute special. "It's got a big emotional wallop at the end, I think they'll be cheering."

Moffat, along with Matt Smith and Jenna Colman, attended the official Doctor Who anniversary celebration at London's ExCel on Friday.

The three-day event, which is being attended by 8,000 fans a day, features appearances from Doctor Who stars from all eras of the series.

Smith told fans during a question and answer session that Steven Moffat's anniversary story "added to the mythology" of Doctor Who.

Moffat described the first ever Doctor Who episode, An Unearthly Child, broadcast on 23 November 1963, as "one of the very best episodes of Doctor Who ever made".

"All the ideas come from there," he said. "The music, the name, the Tardis, the police box bigger on the inside... in terms of brand new ideas that's a rollercoaster of 25 minutes."

Among fans attending the event was Taylor, 13, from London, a fan since the show was revived in 2005.

"It's a big point in Doctor Who history," she said. "I hope it lives up to the fans' expectations. They'll be expecting a big climax."

American Richard LeCour said he made a special trip from his home in California because Doctor Who had been "part of my life for 40 years".

He said he was "very excited" to see how the story would handle the introduction of John Hurt's Doctor.

Reuben Mount, a Matt Smith fan from Brighton, admitted the episode The Name of the Doctor left him "in tears". He added: "I want to know how John Hurt fits into all of it, I want to know the answers."

His friend, Adam Highway, also from Brighton, predicted that Doctor Who had a long future ahead of it.

"It'll go on as long as it keeps that balance of appealing to people who don't know the history, but respects the history for those who give a damn about it. It think Steven Moffat's got it spot on."

The anniversary story is Smith's penultimate outing, before he regenerates at Christmas into a new Doctor, played by Peter Capaldi.