In the conclusion of the four-part story "The Three Doctors," the first, second, and third Doctor (played respectively by William Hartnell, Patrick Troughton, and Jon Pertwee) are still trapped in Omega's black hole, along with the third Doctor's friends Jo (Katy Manning), the Brigadier (Nicholas Courtney), and Sgt. Benton (John Levene). Using their combined energies, the three Doctors attempt to escape their cosmic prison and to prevent the vengeful time lord, Omega (Stephen Thorne), to carry out his terrible campaign of revenge against his own race. Without giving away the ending, it can be noted that actor William Hartnell retired after the completion of filming. First telecast on January 20, 1973, Doctor Who: The Three Doctors, Episode 4 was written by Bob Baker and David Martin. ~ Hal Erickson, Rovi