Today David Tennant, the Tenth Time Lord, will be reunited with two of his favourite and most iconic companions - Sarah Jane Smith and the faithful robot dog, K9.
K9, who was the loyal friend to Tom Baker's Doctor between 1977 and 1981, joins Sarah Jane Smith, the Doctor, Rose and Mickey in a fight against the evil Krillitanes.
John Leeson, the voice of K9, says: "I am delighted to have been invited back on board the series - and a little gathered rust is no object to a fully functional K9!"
Producer Phil Collinson adds: "It is great to be welcoming K9 back to Doctor Who.
"A whole generation fondly remember him as an ever-faithful companion and best friend to Tom Baker's Doctor.
"I hope the new generation of viewers will fall in love with him in the same way. I'm sure he's going to prove an invaluable help to the Doctor in the fight against intergalactic evil."
Elisabeth Sladen says of reviving Sarah Jane: "I am so thrilled to be back and I feel so empowered by the affection that [the whole team] have for Sarah Jane Smith and for the programme.
"Toby Whithouse's script is wonderful - it's an absolutely truthful progression of Sarah.
"I just hope I can live up to their expectation, and I hope I can still run as fast after 30 years!"
The episode, which will be seen next spring in the eagerly-awaited second series, sees the team investigate sinister events at a modern-day comprehensive school, and also stars Anthony Head (Buffy The Vampire Slayer, Little Britain).