Sir Ian McKellen and Andy Serkis, who played Gandalf and Gollum, will give their voices to two characters in a new animated film, called Flushed Away.
It's about a pampered rat who gets washed away from his penthouse flat and has to survive in the sewers.
Rumours also say X-Men star Hugh Jackman is in talks to join the cast, though this hasn't been confirmed.
Flushed Away is being made by Aardman Animation, the team behind Chicken Run and Wallace And Gromit.
It's the second kids film that Sir Ian is involved in, as he's set to give the voice to Zebedee in the new film of 70s cartoon The Magic Roundabout.
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