It means Karen Gillan's reign as the show's most stunning redhead could be over.
Gorgeous Lily, 22, will appear alongside Time Lord Matt Smith and his assistant Amy Pond, played by Karen, in the new BBC1 series later this year.
The episode, set in the 1600s, will see the Tardis landing on top of a pirate ship.
Lily told TV Biz: "It is a great script and a fun character and I am glad to be involved." An insider said: "Lily's a big fan of the show and jumped at the chance to appear. She will play a sea creature in an episode based around a well-known tale. It's a real ocean adventure."
Source - The Sun