Warning: This story contains plot spoilers
By Tom Bishop
BBC News entertainment reporter
A Christmas Day special of sci-fi series Doctor Who contains an anti-war message, as new Doctor David Tennant tackles an alien invasion of Earth.
David Tennant says he "fully intends" to remain in the role
"It's Christmas Day, a day of peace," said chief writer Russell T Davies. "There is absolutely an anti-war message because that's what I think."
Actress Penelope Wilton plays the Prime Minister in the hour-long show.
In one scene she says of the US president: "He is not my boss and he is certainly not turning this into a war."
A later scene echoes former Prime Minister Margaret Thatcher's decision to sink the General Belgrano during the Falklands conflict in 1982.
Wilton's Prime Minister orders the destruction of a retreating alien spaceship, a decision condemned by the Doctor.
"She does that very easy speech about not listening to the American president but at the end she's out of her depth and she does the wrong thing," said Mr Davies.
Entitled The Christmas Invasion, the episode also features a gang of deadly robot Santas and a killer Christmas tree.
The Doctor is hunted by robot Santas and a killer Christmas tree
It is David Tennant's first full episode in the role, as he replaced Christopher Eccleston who left the hit show after one series.
When asked how long he would play the Doctor, Tennant said: "I fully intend to be here this time next year, and let's wait and see what happens after that."
Asked whether Billie Piper would quit her role as assistant Rose Tyler after the forthcoming series, Mr Davies said: "You will have to wait and see."
Tennant said he was having "lots of fun" in the coveted role. "It's a thrill to be part of it," he said.
"It is like no other job in the world - you are sword-fighting one day, swinging off ledges on another. It never fails to surprise and delight."
The new series will feature a brief kiss between the Doctor and Rose. "There is a lot more of that to come but we don't like to give anything away," said Mr Davies.
Penelope Wilton (left) plays the UK's Prime Minister in the special
For the first time in the show's history, the previous series encouraged viewers to empathise with murderous villains The Daleks.
Amid the action and humour in the forthcoming series, would viewers be asked to sympathise with returning monsters The Cybermen?
"There is a moment with them - it is a very 'feely' show," Mr Davies said. "We want to treat them as real, to discover why they are the way they are."
Long-term Doctor Who fan Tennant added: "Cybermen are part human, in a way. We want to look into that."
Doctor Who: The Christmas Invasion will be screened on BBC One on Christmas Day at 1900 GMT.