Media playback is unsupported on your device Media caption Doctor Who stars Matt Smith and Karen Gillan reveal what is in store for the new series

Doctor Who boss Steven Moffat has said the new season of the sci-fi show kicks off in a darker style than usual.

Launching the first two episodes at London's Olympia, Moffat pleaded with journalists to keep the show's secrets under wraps.

The opener sees Matt Smith's Doctor reunited with Karen Gillan's Amy Pond, Arthur Darvill's Rory and Alex Kingston's River Song in 1960s America.

The quartet team up with President Nixon to fight a new alien threat.

The aliens - partly inspired by Edvard Munch's The Scream - are introduced in the two opening episodes, The Impossible Astronaut and Day of the Moon.

The programmes, which feature scenes shot in the Utah desert last year, will be shown on BBC One later this month.

"To be honest, it's darker than any other opener of a season," said Moffat, who wrote both episodes.

"We've been pretty dark before in Doctor Who. But we're coming in from the dark side just because we haven't done it that way before."

Yet Moffat, who took over as lead writer and executive producer, said he did not think the shows were too scary for children.

I've got two kids of my own and I'd never do anything I didn't think was acceptable for them Steven Moffat

"First of all, you make Doctor Who frightening to appeal to children. It's the children who find it frightening, not the adults.

"Children absolutely rank Doctor Who stories in order of frightening-ness - that's what it's about.

"You put the jokes in and the silly bits in for the adults and you put the scares in for the kids. I've got two kids of my own and I'd never do anything I didn't think was acceptable for them.

"Having said that, one of them does tend to sleep on our bedroom floor."

Future episodes will feature such guest stars as model Lily Cole, Downton Abbey's Hugh Bonneville as a pirate and ex-Coronation Street actress Suranne Jones.

The latter will appear in a story called The Doctor's Wife, written by fantasy author Neil Gaiman.

'Tease, don't tell'

For the first time this year's season is being split in two. The first seven episodes will air from 23 April, with the final block of six broadcast in the autumn.

"If you run for 13 weeks you can start to feel as though you can miss one episode and it'll be okay," Moffat said.

"We don't want that feeling. We stop for a few weeks and let you all worry about what's happening and then come back."

Moffat went on to ask journalists to avoid spoilers, saying it would be "a lot more fun for the kids" if nothing was given away. "Tease them but don't tell them," he pleaded.

And asked about plans for Doctor Who's 50th anniversary in 2013, he would only say: "Yes, there are thoughts..."