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.

The Doctor and Nyssa, aided by Ambril's assistant Chela, locate Ambril's aged predecessor Dojjen, who predicted the Mara's rebirth before wandering off into the wilderness. The Doctor allows himself to be bitten by a snake in order to enter a state of mental commune with Dojjen, who tells him that fear is the only true venom and that in order to defeat the Mara he must find the still point within himself.

The Doctor and his friends then return to the caves where the ceremony is being held. The Doctor, by concentrating his thoughts with the aid of a small replica of the great crystal, is able to find the still point and repel the Mara.