When Doctor Who lands in the nation's living rooms on Christmas Day it is traditional for a succession of baddies to follow. This year, however, his foe won't be Daleks and Cybermen – but an extremely hungry, flying shark.

And if that isn't unlikely enough, the shark features in an adventure that takes Charles Dickens' A Christmas Carol for its inspiration and involves the Doctor trying to save a ship of 4,000 passengers from certain death.

"If you're going to do a Christmas Day episode, which is based on the principle that the audience have had a selection box for breakfast and are probably drunk, then you have to move it on a bit – because a normal episode of Doctor Who wouldn't be enough," joked lead writer Steven Moffat after a special preview screening last night.

The show marks Matt Smith's first Christmas Day special as The Doctor. "I've always wanted to do a Doctor Who Christmas special. It's wonderful, It's been wonderful," he told fans. The episode also stars Michael Gambon as the scrooge-like Kazran Sardick, the singer Katherine Jenkins – and of course the Doctor's companion Amy Pond.

Away from the snapping jaws of a giant floating shark, which was codenamed Clive during filming to stop the surprise getting out, the show has, at its heart, a simple love story. "I think [Christmas shows] have to be emotional," Moffat said. "You have to go on the whole journey – laugh and cry a bit."

Jenkins, for whom Doctor Who is her acting debut, has been on a journey herself. Despite her musical stardom, she counts playing the role of Abigail Pettigrew in the Christmas programme as the most nerve-wracking thing she's done.

"The first day I was filming for Doctor Who we did a night shoot, and I'm normally the chattiest person going; you know I'm nervous when I can't speak. And I spent the entire day so so quiet," Jenkins said, admitting she found not telling anyone she had been cast in the show difficult.

"I thought I had to keep the secret right up until Christmas day and just put the telly on and say 'Mum! You might want to watch that!'"

Last Christmas, Matt Smith, who made his Doctor Who debut in a New Year special, said he watched the show with his mum and dad. "I was just sort of going 'Oh dear Lord I'm going to turn into Doctor Who'. It was quite an odd experience, it was exciting," Smith said. "I was just thinking 'Please don't hate me, everybody! Pleease.'"

The star says that the fame that has come with Doctor Who has meant that he is more recognisable. "But it's only as drastic as you make it, and you can still go about all the things you do quite happily, I think – you've just got to wear more hoods."

• Doctor Who: A Christmas Carol will be broadcast at 6pm on Christmas Day