David Tennant is to appear as the Time Lord in Doctor Who spin-off The Sarah Jane Adventures, the BBC has announced.
Tennant, 38, will appear over two episodes of a new 12-part series of the CBBC show, which starts in September.
Russell T Davies, executive producer of The Sarah Jane Adventures and Doctor Who, said it would be a "full on appearance" and "not just a cameo".
The show follows the adventures of the investigative journalist Sarah Jane Smith - a former Doctor's companion.
Smith is played by actress Elisabeth Sladen, 61, who also played the character in Doctor Who opposite Jon Pertwee and Tom Baker between 1973 and 1976.
Sladen said she was "absolutely over the moon" to be co-starring alongside Tennant.