Matt Smith is to guest star in the new series of The Sarah Jane Adventures, the BBC has announced.
In a special two-part story, The Doctor will be reunited with Sarah Jane (Elisabeth Sladen) and fellow former companion Jo Grant (Katy Manning).
The script has been penned by Russell T Davies, executive producer on Sarah Jane and former Doctor Who showrunner. The episodes will mark the first time that Davies has written for the Eleventh Doctor.
"It's a fantastic script and I can't wait to work with another Doctor and hope Matt has fun with us," said Sladen. "I've known Katy for ages and I am delighted to be working with her. I last met her in LA but this time we will be in Cardiff. LA was good but Cardiff is better."
Manning, who appeared alongside the Third Doctor from 1971 to 1973, said: "Playing Jo Grant again is something I never really considered. I was gobsmacked when they told me and I am over the moon. What an incredible little treat. I come home [to the UK] and this is one of the first things that happens."
Nikki Wilson, co-executive producer on the Who spinoff, said of the two-parter: "Viewers are in for a real treat, with an action-packed story, full of Russell's usual wit and warmth, which takes the gang inside a secret base beneath Snowdon and introduces brand new vulture aliens, the mysterious Shansheeth. All this, plus a trip to an alien planet - a first for The Sarah Jane Adventures."
Sarah Jane was recently recommissioned for a fourth and fifth series, to air in autumn 2010 and 2011 respectively.
Meanwhile, other guest stars for the series will include Laila Rouass, Cheryl Campbell and Harry Potter's David Bradley as the voice of the Shansheeth.
I've yet to see a so-called "crossover episode" of any series that wasn't totaly embarassing. David Tennant's appearance in a previous Sarah-Jane was completely superfluous, the episode would have played better without his character being in it.
