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.

The ghostly figure of the Watcher had a white, featureless, unformed face and body and white clothes. Though he used the Doctor's TARDIS at one point, he apparently had the power to traverse time and space on his own.

He 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, the Watcher was capable of 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 companion Adric. 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.

At the moment of the Fourth Doctor's death, the Watcher appeared and merged with the Doctor to create the Doctor's next incarnation.

_{Biography from the TARDIS Data Core article, licensed under CC-BY-SA}