The gang is on the trail of a stolen Axtakl artifact called the Ancient Astronaut.
Scooby and the gang are at the Museum of Ancient Art, when an Egyptian sarcophagus begins to cough and smoke. The curator, Mr. Hadley, arrives to extinguish it, but it turns out to be a self-destructing tape recorder. Meanwhile, thieves have made off with an Axtakl artifact called the Ancient Astronaut. Clues point to the thieves returning with the artifact to Axtakl, so the museum offers to fly the gang down to Central America to retrieve it.
There, they find the Ancient Astronaut on top of the largest pyramid temple. Fred notices the nose of the ship is pointing at a small lake where they see two men in a small raft. They climb down from the temple, but the men are nowhere in sight. As Scooby swims out to the raft, two men in diving suits surface. Scooby subdues the attackers. Fred and Shaggy tie them up to be turned over to the Axtakl authorities, who hire the gang to dive and search for the treasure the thieves were after.