Thalia and Dalbiac are holding Lulli and Dr. Winthrop prisoner in a remote windmill. Thalia plans to use the deadly H.23 gas on them to prove its effectiveness to a potential buyer. But when Tarot arrives he is welcomed with the gas instead. The nightmare begins...

Tarot is saved from being burnt at the stake by the Roundheads, but Thalia and Dalbiac are waiting with more nightmare gas. And this time there's no escape.

A diamond that can turn people in to chalk; a talking computer and an eccentric chess master called Ceribraun involve Tarot in a new and interesting mystery.