Leah chats to Sarah Jane actress Elisabeth Sladen
David Tennant's special role on the Sarah Jane Adventures helped bring more than a million people to the show.
A whopping 1.3 million people tuned into Thursday's episode of the programme, in which David Tennant appeared as the Doctor near the end.
That means one in every 10 people who were watching television at the time were tuned into the CBBC show.
And David Tennant isn't finished yet - he's back to star in the second part of the story on Friday afternoon as well.
David's next outing as Doctor Who will be in an episode called The Waters Of Mars in November and his last appearance will be in a big two-part story over Christmas.
On Friday lots of journalists were allowed to watch the Waters Of Mars for the first time. Everyone else will get their chance to watch it on 15 November.
You can watch David on the Sarah Jane Adventures on Friday at 4.35pm on CBBC on BBC One.