Watch Doctor Who: The Mutants (1972) Official Trailer

The Time Lords send the Doctor and Jo on a mission to deliver a sealed message pod to an unknown party aboard a Skybase orbiting the planet Solos in the 30th century. They are caught quickly in a power struggle between the cruel Marshal of Solos and the young Solonian Ky over the future of Solos - a future that hinges on the contents of the message.