Tegan falls once more under the influence of the Mara and directs the TARDIS to the planet Manussa. There, the Federator's son Lon and his mother Tanha are preparing for a ceremony to celebrate the banishment of the Mara five hundred years earlier. The Mara takes control of Lon and uses him and Tegan to obtain from Ambril, the Director of Historical Research, the 'Great Crystal' - the large blue stone that originally brought it into being by focusing energy from the minds of the planet's one-time inhabitants. The Mara now plans to use the crystal during the ceremony to bring about its return to corporeal existence.
Peter Davison
as The Doctor
Sarah Sutton
as Nyssa
Janet Fielding
as Tegan Jovanka
Martin Clunes
as Lon
Colette O'Neil
as Tanha
John Carson
as Ambril
Preston Lockwood
as Dojjen
Jonathon Morris
as Chela
Brian Miller
as Dugdale
George Ballantine
as Hawker
Brian Grellis
as Megaphone Man
Barry Smith
as Puppeteer
