Robot dog K9 will be reunited with Doctor Who in the new series after 24 years apart, say the show's producers.
Original voice of K9 John Leeson returns in the new episode
K9, who helped Tom Baker's Time Lord between 1977 and 1981, will join new Doctor David Tennant for one episode.
The show, to be screened on BBC One next spring, sees the team investigate sinister events at a modern-day school.
It will co-star Buffy the Vampire Slayer actor Anthony Head and Elisabeth Sladen, who is returning as 1970s Doctor's assistant Sarah Jane Smith.
K9 and Sarah Jane will join the Doctor and his current sidekicks, Rose and Mickey, in a fight against the evil Krillitanes.
Producer Phil Collinson said it was "great" to be welcoming back K9.
"A whole generation fondly remember him as an ever faithful companion and best friend to Tom Baker's Doctor," he said.
"I hope the new generation of viewers will fall in love with him in the same way."
John Leeson will return to provide the voice of K9.
"I am delighted to have been invited back on board - and a little gathered rust is no object to a fully functional K9," he said.
K9 was designed as a robotic pet by a Professor Marius, who left him to the Doctor.
A total of three versions of K9 were built. The Doctor sent the third one to companion Sarah Jane, who had left in 1976.
Tennant inherited the Doctor Who role at the end of the last series
Sarah Jane and K9 appeared together in the spin-off show K9&Co in 1981 and the 1983 anniversary special The Five Doctors.
Revived after a 16-year absence, Doctor Who proved to be a ratings success on its return in March.
A 13-part series, including the K9 episode, will follow a Christmas special.
The Cybermen will return in the new series, which will also see the Doctor and Rose meet Queen Victoria and an evil race of Cat Women.
One episode will be written by comedian Stephen Fry, while The League of Gentleman star Mark Gatiss will once again contribute to the writing.