When the Doctor had to turn himself human to avoid the Family of Blood, he placed his Time Lord consciousness and true physiology inside a small ornate fob watch, which he then kept nearby at all times while he lived the life of school teacher John Smith. Although he had no memory of ever being the Doctor, and thought of himself as truly human, when the watch was opened, all the real 'Doctor' flooded back into him, turning him once more into the Time Lord he actually was.
The Doctor's oldest, bitterest enemy, the Master - another Time Lord - also had a fob watch and after escaping the devastation of the Great Time War, he too created a human persona for himself - the kindly professor Yana. When he opened his fob watch however, the Master was back, ready to take on the Doctor and threaten the very existence of the universe.
This fully working fob watch has detailed engraving on the case and features speech, light & sound FX and watch chain. Requires 4 x LR44 batteries, included.