Several friends from the Doctor's past will be back at the end of the new series of Doctor Who.
At a special launch event last night, a trailer was shown featuring some of the old faces who will return.
They include Martha, Capt. Jack, Sarah Jane, Mickey, Jackie, Rose, and Harriet Jones.
Who they'll be fighting is still a mystery, but the person who might be the main villain was shown partly hidden in shadow.
The villain had the bottom half of a Dalek, while its top half looked like it could be human.
David Tennant, who finished filming the series last week, told CBBC Newsround: "It's been great to have a lot of old friends back. We had a really good time."
The team behind the Doctor Who show will be hoping that the return of popular characters like Rose and Sarah Jane will mean that millions of viewers will keep tuning in.
Tennant added that he thought the storylines in this series would also be an important factor.
"Hopefully the stories are still interesting and exciting and different and unexpected," he said. "I can't wait for people to see it."
In this 13-episode series the Doctor meets his daughter, battles aliens called Sontarans, and investigates a creepy library.
His new companion is Donna, who he teamed up with in the 2006 Christmas Special, The Runaway Bride.
Catherine Tate says Donna is a very different companion because she "doesn't hero worship him. She doesn't fancy him".
But as the series goes on Donna "listens more to him, and you get to see a much more vulnerable side to her".
The new series of Doctor Who start this Saturday at 6.20pm on BBC One.