During a holiday on the Dutch rural B&B, Suske finds, while diving for Shanulleke, a black diamond. He gives it to Sidonia, who breaks her promise to proudly hold on to it after the local Baron Roger de Lacheloze offers her a hefty price. Their host Alwina tells the legend whoever profits from the stone shall perish by the next new moon. Sidonia gets sick and unable to laugh, the fatal symptom. Barabbas's time-machine sends Lambik, Alwina and the kids to the 16th century, when the legend started, at a magician's tower, to have it destroyed. They soon fall prey to feudal sorcery charges, 'solved' by fatal 'divine ordeal'. Even in they survive that, there's a mysterious black knight after them and the stone.