An Edwardian yacht in deep space races around the planets. There is a double agent in the TARDIS crew. The White Guardian warns the Fifth Doctor of great danger. Turlough must finally choose sides and at the end of the race lies the prize of Enlightenment.
| Actor | Role |
|---|---|
 Peter DavisonThe Doctor | The Doctor |
 Janet FieldingTegan Jovanka | Tegan Jovanka |
 Mark StricksonVislor Turlough | Vislor Turlough |
 Cyril LuckhamThe White Guardian | The White Guardian |
 Valentine DyallThe Black Guardian | The Black Guardian |
 Keith BarronStriker | Striker |
| CChristopher BrownMarriner | Marriner |
 Lynda BaronWrack | Wrack |
 Leee JohnMansell | Mansell |
| JJames McClureFirst Officer | First Officer |
 Tony CaunterJackson | Jackson |
 Clive KnellerCollier | Collier |