Simon, Henry, and Jack Barker decide to ignore their parents' warning to stay away from Superstition Mountain in order to search for their missing cat Josie, but they begin to see the wisdom of their mom and dad's advice when they find three human skulls.

The Barker brothers and their friend Delilah secretly climb up to Superstition Mountain one last time, hoping to solve the remaining mysteries, including whether the librarian is really the ghost of Julia Thomas, what was their uncle Hank's role in discovering the gold mine, and especially, who is trying to kill them.