The TARDIS arrives on the planet Manussa—much to the Doctor's surprise, because Tegan has mysteriously set the co-ordinates. But Tegan, once again a member of the TARDIS crew, is not her own boss. An unsuspecting medium for the sinister Mara, she enables the evil exile to return to his home planet. On Manussa the ten-yearly celebration of the Mara's banishment is about to take place. Only the Doctor realises that this could in fact mark the spectacular revival of a reign of terror—but no one will heed his warning.