Doctor Who stars David Tennant and Billie Piper flicked the switch on the Cardiff Christmas lights on Thursday.
The pair, who play the Doctor and his sidekick Rose Tyler, are preparing to star in a special episode of the show on Christmas day.
David Tennant will be making his first appearance as the Time Lord, making him the 10th incarnation of the character.
Billie let slip her favourite scene in the special programme involves her being attacked by a Christmas tree.
"The Christmas tree attacks us. That's pretty exciting and quite threatening and scary at the same time," said Billie.
She also revealed to the crowd that other dangers the pair will face in the new series include a werewolf, as well as the cybermen.
The first and second series' of Doctor Who are both filmed in Cardiff.