[The Forbidden Tower] continues from The Spell Sword; the alien Catmen have been vanquished, though at heavy cost. Damon Ridenow and Andrew Carr marry the twin sisters Callista and Ellemir of the house of Alton. Both men are displaced persons. Andrew has given up all his ties to Earth and is feeling his way in a new culture. Damon has been denied his vocation as a Keeper, one of the highly trained telepaths who work in the Towers. The ancient science of the Comyn lords of Darkover centres rigidly upon the training and discipline received in these towers and is hedged with prohibitions.

The force of these taboos falls most heavily on Callista, who renounces her vows as a Keeper, trained in the Tower of Arilinn, to marry Andrew, the man who saved her psychically and physically from the aliens…. Keepers, through their psychic power and their link...