Who and why needed to throw two strange glowing stones into Michel's room? And where did they soon just as mysteriously disappear? Do such stones exist in nature or is it a clever trick? There are even more questions when Michel and his friends discover a path not far from the Devil’s Spring, lined with the same “fireflies.” And when they find out where this path leads, then...

The story was also published under the title “The Secret of the Devil’s Spring.”