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.

• BBC
• Peter Davison
• Janet Fielding
• Mark Strickson
• Cyril Luckham
• Valentine Dyall
• Keith Barron
• Christopher Brown
• Lynda Baron