The Watcher (Logopolis)

The Watcher was a manifestation of the Doctor who appeared during the events that led up to the regeneration of his fourth incarnation into his fifth. He appeared to the Doctor and his companions, and brought Nyssa to the planet Logopolis.

Appearances
In a dream, the Fourth Doctor saw the Watcher and thought of his next incarnation. (PROSE: Into the Silent Land)

The Watcher silently observed events on Earth, particularly the events leading to Tegan Jovanka entering the Doctor's TARDIS. It was around this time that the Doctor first noticed the Watcher. Although apparently silent, (TV: Logopolis) the Watcher was capable of telepathic communication. He communicated with the Fourth Doctor, and was seen apparently carrying on a conversation of some sorts, though what was said was unknown. Later, he communicated with the Doctor's companions Adric and Nyssa, who thought his voice seemed familiar. (PROSE: Logopolis) The Watcher, apparently on his own accord, transported Nyssa from her home planet of Traken, a planet which would soon die in the entropy wave, to Logopolis and safety. Later, apparently working on the Doctor's instructions, he piloted the TARDIS from Logopolis to keep Adric, Nyssa and Tegan safe. It was during this voyage that the Watcher engaged Adric in a private conversation; (TV: Logopolis) when trying to recall the Watcher's voice, Adric found he could only imagine the Doctor's. (PROSE: Logopolis) As the Fourth Doctor died, the Watcher appeared and merged with the Doctor to create the Doctor's next incarnation. (TV: Logopolis)

During his own regeneration, the Fifth Doctor again saw and merged with a recreation of the Watcher that reflected the Sixth Doctor in his mindscape. (TV: The Caves of Androzani, AUDIO: Winter)

Shortly before his regeneration, the Tenth Doctor saw a solitary pale watcher observing him; shortly after the TARDIS dematerialised, the watcher also disappeared. (PROSE: The Advent of Fear)

Appearance
The Watcher was a ghostly figure with a white, featureless, unformed face and body. He wore white robes. Though he used the Doctor's TARDIS at one point, he apparently had the power to traverse time and space on his own. (TV: Logopolis)

Behind the scenes

 * The nature of the Watcher's conversations with the Doctor and Adric was never revealed. Logopolis suggests that the Doctor was made aware of his impending regeneration due to his statement. "It's the end, but the moment has been prepared for." The knowledge passed to Adric is never revealed, although after the transformation, Adric does show an unexpected knowledge of regeneration in Castrovalva.
 * Several other "partial Doctors" have appeared in the history of the series, most notably an evil manifestation, the Valeyard. (TV: The Trial of a Time Lord) In the audio stories Trial of the Valeyard and He Jests at Scars..., the Valeyard is identified as the first known fully independent Watcher.