A ghost ship off Chiloé Island and the whispering moai of Easter Island point Indiana Jones toward the same impossible destination: a hidden world inside the Earth, populated by a civilization no surface map accounts for. Indy's own carelessness with a dangerous relic on an earlier trip helped tip the balance between the two worlds, and now an ambitious warlord inside the hollow earth means to use that imbalance to break through and conquer the surface. It falls to Indy to set things right before the interior world's army arrives.
Summary by Loreful, from public sources