During the II crusade in Palestine, Thunder Captain discovers in the dungeons of a fortress a moribund Christian called Juan de Ribera. He entrusts Thunder Captain with giving back a magical chalice to Spain. Juan de Ribera asserts it is the Holy Grail, which was stolen from a millenary order. Thunder, besides his inseparable friends Crispin and Goliath and a Viking princess called Sigrid, will have to face an evil feudal lord and his black “devils”, which have frightened all the population.