The actor who plays the lead character in Dr Who has warned against watching the episode leaked on to the internet.
Christopher Eccleston told CBBC Newsround the film is not the finished product and could ruin future episodes.
"It's a rough cut without any special effects. Wait until it's broadcast and see it in all its glory," he said.
The episode of the sci-fi programme, which also stars former pop singer Billie Piper, was leaked two weeks before it's due to be on TV.
A BBC spokesman said they were investigating how the leak happened.
The BBC has spent several million pounds making a new series of the show, which first started in 1963. It was last seen on television nine years ago.
The leaked episode appears to be called Rose - after his new sidekick played by Billie Piper - and it lasts 45 minutes.
'Not the final version'
Fans think it is the first episode of the new series, but Eccleston warned them not to take it seriously.
"Don't watch it because it's not the final version. It will spoil the first episode," he told CBBC Newsround.
Dr Who is the longest-running sci-fi series ever on British TV, and Christopher Eccleston is the ninth person to play the Dr.
Paul McGann played the title role nine years ago. Dr Who has been voted the show most people want to see come back on TV.