David Tennant's final scenes as Dr Who are a "brilliant send-off" - that's according to his co-star John Simm.
The actor's returning as the Doctor's nemesis, The Master, in a two-part Christmas special which ends David Tennant's run as the Time Lord.
He said filming the episodes, which is now completed, was really good fun.
"It'll be a brilliant send-off for Mr Tennant. Last time I did it it was such fun. It was wonderful to be asked back and to be in the very, very last one."
John Simm said one of the reasons he wanted to be on the show was so his son Ryan could watch him.
He explained that most of the TV programmes he's in are too adult for his eight-year-old to watch.
"It was a great opportunity to make my son proud," he said.
The next series of Doctor Who, starring Matt Smith, is set to be on screens in spring 2010.