Dougal's tug is hired by two men posing as divers searching for a Great Lakes wreck but who are really after stolen jewelry cached on the lake bottom by the thieves after a Montreal robbery.

Billy and Pete bypass the police and conduct their own investigation into a case of vandalism of the Williams' lodge, which leads to a dangerous midnight confrontation.

Those two intrepid fishermen, Billy and Pete, hike into the mountains to visit their secret fishing place - a jewel-like lake filled with fine pike. To their amazement, they find that the fish are dying and it appears that bush pilot Dennis Mogubgub has something to do with it. Horrified, the boys try to trap and remove the fish to another lake before it's too late to save them. That's when they learn a lesson in conservation.