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.
Peter Davison
as The Doctor
Janet Fielding
as Tegan Jovanka
Mark Strickson
as Vislor Turlough
Cyril Luckham
as The White Guardian
Valentine Dyall
as The Black Guardian
Keith Barron
as Striker
Christopher Brown
as Marriner
Lynda Baron
as Wrack
Leee John
as Mansell
James McClure
as First Officer
Tony Caunter
as Jackson
Clive Kneller
as Collier
