Melusine, a deity of rivers and seas, long ago lost the Battle of the Betrayal and is imprisoned in a nexus centred on the source of the Lynn River. She can only travel in the flowing water of her river, and cannot set foot on her banks. Her cats are not so bound. Neither pets nor familiars, they move freely between the realities, her agents among the humans who live along the Lynn.
When Melusine's enemies come to the valley in search of revenge, she needs to rebuild the war band she has lost over the centuries. Her cats are the keys to this. Each one is a guardian, and formidable in its greater avatar. A guardian, along with two chosen warriors, form a triad. All three must come into the triad willingly, and as willingly give their oaths of loyalty to her. But Melusine is both prisoner and prison-keeper, and now must gather triads to guard that other dungeon at all costs.
Hal Rayner enjoys his job at Greenlynn's Black Dog pub. He gets on well with the locals, and has been adopted by the usually standoffish pub cat. Then the dreams start, a red-headed stranger turns up, and before he knows it, Hal is caught up in other-worldly danger and weirdness. The cat, Tawny, is right in the middle of it, and Hal can't understand how a common or garden mog can become something out of a prehistoric nightmare.
Gryffyth, the son of Nodens Silverhand, has been released from long imprisonment, and despite the unhealed wound given to him by Melusine in that distant battle, the years haven't blunted his hunger for revenge. He is still loyal to Gronw, and when Morgan sends for him, he willingly gives his aid. Gryff is determined to find and free his imprisoned liege lord. Then he meets Hal and Tawny, and suddenly the rules have changed.
