Newsround went behind the scenes during the filming of the second episode of Doctor Who - The End of the World.
Tim Masters reports from a space station five billion years in the future (somewhere in Cardiff!).
Almost as soon as I arrive outside the Doctor Who set in central Cardiff, a minibus pulls up and out jump several alien creatures that I can only describe as "tree people".
As they trudge through the mud, gathering up their robes, it's clear this is going to be a very unusual day.
Jabe - a tree woman
The inside of the building, the Temple of Peace, has been transformed into a stunning space hotel. The main hall is flanked by huge marble pillars and at one end there is a massive window.
Throughout the hall are glass cabinets containing old relics from Earth's history. One contains a battered old copy of Harry Potter and the Philospher's Stone.
On set, the Doctor (played by Christopher Eccleston) and Rose (Billie Piper) are rehearsing a scene before the cameras roll.
They are discussing the destruction of Earth by the sun. A line of dialogue from Rose stands out: "But hold on, they did that once on Newsround Extra - the sun is expanding, that takes hundreds of years."
Wow - Newsround is still getting talked about in five billion years' time!
The Face of Boe
Doctor Who producer Phil Collinson gives me a quick tour of the set in between takes. "We've got all the monsters here today, more than ever before."
And he's not joking. As well as the tree people, there's a huge animatronic head in a glass tank (the Face of Boe), blue-skinned aliens, and a flat piece of skin with a face on it - Lady Cassandra - the last human.
Phil's job as producer is to oversee the production from script to screen. He says one episode takes two weeks to film - and there are 13 episodes in all.
Lady Cassandra - the oldest human
Billie Piper, who plays 19-year-old Earth girl Rose, tells Newsround that she's keen not to focus on what's been done on Doctor Who in the past.
"This is a whole new approach," she says. "This is about the Doctor and Rose and friendship - that's the core of every tale."
The end of the Earth
I sit at one end of the set while the director Euros Lyn gathers a huge number of extras in the hall and explains a bit about the story.
"You are the servants in the most expensive hotel in the universe," he says. "The richest people in the universe have gathered to watch the destruction of the Earth."
It's a major job getting everyone to move into the right positions on cue. There is a group of children from a local drama school who are playing alien servants.
Out in the lobby, assorted aliens are having their make-up touched up before the cameras roll for the big entrance scene.
Moxx of Balhoon
Now there are even more tree people, a small blue alien with a huge head called the Moxx of Balhoon, and strange bird creatures with heads like pterodactyls.
My only regret is that there's no sign of the Doctor's greatest enemy - the Daleks! They are destined to return in a future episode...
Doctor Who is on Saturdays on BBC One at 7pm.
Check out the Newsround Showbiz Doctor Who special on Saturday at 3.50pm on the CBBC Channel.