In the conclusion of the four-part story "
Enlightenment," the Doctor (Peter Davison) races against the evil Captain Wrack (Lynda Baron) to win a yachting competition in space, the prize of which would prove disastrous should it fall into the wrong hands. The Doctor's old enemy the Black Guardian (
Valentine Dyall) attempts to determine the race's outcome, but he eventually faces defeat from an unexpected source. With this episode, Mark Strickson officially becomes a regular in the role of Turlough. Written by Barbara Clegg, "Enlightenment, Episode 4" first aired on March 9, 1983. ~ Hal Erickson, All Movie Guide