Steve Grendon and Sim Jones planned, now that June days of freedom had finally arrived, to build themselves a raft and go down the Wisconsin on an exploring trip. They assumed that they would be able to talk their parents into approving the expedition, but decided they would worry about that later. Actually theirs wasn't just any old exploring trip. They had a very definite plan. They were going to hunt for treasure, the lost Winnebago treasure which Steve had read about in a history book and which might still be hidden in the cave of Bogus Bluff...

Trouble always seems to find Steve Grendon and Sim Jones. This time, they are far from home on an island right smack in the middle of the Wisconsin Dells they thought was deserted. Sim is practical, logical, and doesn't like surprises. Steve is imaginative, carefree and—truth be told—the one who typically invites the trouble. They just wanted to get away for some camping and fishing, to do something besides let their summer vacation slip away from them. But they didn't expect to encounter the ghost of an Indian chief who made it very clear he did not want them on his island! Undeterred, the boys quickly find themselves on the trail of the ghost and, before they know it, discover they are the ones being hunted. Fire Bear, a young Winnebago Indian the boys meet on their trip, may be able to help them solve the mystery. But whom do you trust so far from home when being hunted by a ghost?

Steve & Sim team up for another mystery in Sag Prairie, Wisconsin. The Brooks' Stock Company has set up their Tent show in the same place in town for years. This year, however, someone is trying to get them out of town. Steve, Sim and their friend Pete put their head together to solve the mystery.

As Steve and Sim stepped out of Joe Brilliot's borrowed cabin to go ice fishing, they found Sepple Bollinger, the town hobo, actually abiding by the law—fishing in season. This was suspicious in itself, and steered the boys straight into a dangerous "out of season" hunt which nearly cost them their lives.

On a canoe trip, two boys stumble across a mysterious house that leads them into the path of danger.

The boys find a youth tied up in an abandoned mill who tells them that he is a visiting prince and has been kidnapped from his Chicago hotel. Steve goes to the Windy City to investigate.