While anchored in Fossil Bay, Gary comes across a wooden mask floating on the water. Mysterious powers of the mask take Gary to scenes from the past leading The Island Gang to sail north into the Gulf Islands. On Saturna Island, they stumble onto a very old estate where a near life size replica of the Blue Diamond, a twin masted schooner, has been built right inside the mansion on the estate.
Carvings from the deck of the schooner have originated in the Native American village of YanYan. Determined to find YanYan, they change their course, heading deeper into the Gulf Islands. While underway, Gary yields to the beckoning of one mask one too many times and is taken into a world from which he has no escape.
The Island Gang learns that YanYan has attracted some associated with the Underground. They must once again outwit their foes as they work to find Gary and bring the Underground to justice.