Christmas is just around the corner, and something strange is happening in the Kirrin woods. After a fall, Dick encounters a mysterious stranger who helps the Famous Five and then disappears again. They discover that he lives deep in the forest. At the same time, three men seem to be on the stranger's trail. Meanwhile, countless protected ferns are disappearing from the woods. Could the herbalist Agnes have an explanation? However, she seems to have her own secrets. In the cold winter forest, the adventurous children follow several hot leads, make some daring decisions, and manage to prevent a major disaster at the last moment.