Ethan and Matt are staying at a lakeside cabin over a summer weekend when they meet Ms. Davenport, the owner of the cabin. She tells the boys about the legend of Dead-end Island, a place where something of great value was hidden by her family many years ago in the caves found there… caves that are dangerous and possibly deadly.
After a showdown with two local bullies, the boys are helped by Ms. Davenport’s grand-daughter, Karen, who later tells them that she found a mysterious key and a clue which talks about a map to the diamond’s location.
After the trio finds the map, they set off on this adventure truly believing they might actually find something of great value. Their adventure leads them to Dead-end Island and into the caves, with the two bullies not far behind. Can Ethan and Matt conquer their fears, escape the bullies, and finally find The Heart of the Island?