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
The Doctor
Sarah Sutton
Nyssa
Janet Fielding
Tegan Jovanka
Martin Clunes
Lon
Colette O'Neil
Tanha
John Carson
Ambril
Preston Lockwood
Dojjen
Jonathon Morris
Chela
