Time Lord No.5 Peter Davison has returned to the show that made him famous, 23 years after finishing his stint in the cult show.
Davison and tenth Doctor David Tennant will appear together in a special episode called Time Crash.
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Who's Who: The tenth Doctor Who, David Tennant, is joined by the fifth Doctor, actor Peter Davison in a specially filmed episode in aid of BBC's Children in Need
The pair teamed up for the special, which will be screened as part of the BBC's Children in Need charity night, in a programme which sees the two doctors team up to thwart an alien.
Veteran actor Davison, 56, who played the Doctor from 1981 to 1984 admitted he was tongue-tied when he first met 36-year-old Tennant on the set.
He said: "I only found out afterwards that he was a fan of my Doctor, because he sent me a very nice text saying that he was at first a bit tongue-tied.
"We were feeling the same, in a way, for different reasons.
"But it was enormous fun working with him, because I go quite fast when I'm acting - it's quite energetic, and he is as well."
Davison succeeded Tom Baker (the Fourth Doctor), at the age 29. He was and still is the youngest actor to assume the lead role.