The David Tennant Treat 4 Today is a tribute to Elisabeth Sladen who sadly died yesterday.
After she had appeared with the third and four doctors (Jon Pertwee and Tom Baker) in the 1970s, Russell T Davies brought her back to star in David Tennant's episode School Reunion.
She also appeared in his episodes The Stolen Earth and Journey's End, plus his final episode The End Of Time Part Two.
RTD also made the spin-off series The Sarah Jane Adventures and David Tennant's final filming as the Tenth Doctor was in the two episodes in that series called The Wedding Of Sarah Jane Smith.
The David Tennant Treat 4 Today is a video of the final part of that episode. When it first aired lots of fans reported getting tearful at the part where Sarah Jane asked The Doctor if she would ever see him again.
That scene carries an even more powerful emotion now.