I saw the scaly sea rise and flood the land, the Red Wanderer like a bloody gash in the sky. That is when they will come, with the rising o' the red comet the Fairgean shall come ...

Before his death, the blind prophet Jorge had foreseen the conjuring of a tidal wave by the sea-dwelling Fairgean, Eileanan's bitterest enemies. Driven by ancient hatreds, the Fairgean have called upon dreadful powers to aid them in their quest to totally annihilate all those who dwell upon the land.

As the time of the comet draws ever closer, Lachlan and Iseult try desperately to avert the terrible fate the seer had prophesied, testing their strength and love to the utmost. Meanwhile, Isabeau must travel a dark and dangerous road to fulfill her own destiny and prove herself worthy of the ring of the sorceress. Facing old enemies and new, the most difficult challenge of all will be conquering the ghosts of her past. For if Isabeau is to heal the land, she first must find the way to healing her own troubled spirit ...

The Fathomless Caves brings to a dramatic conclusion the war between the land-dwellers of Eileanan and the sea-dwelling Fairgean – a war which has been building steadily since readers first found out about the Fairgean's devious plans in Dragonclaw.