American history teacher John Butler along with his wife Kim and their two children Katie and Greg, as well as their dog Digger, are on a rafting trip on the Amazon river. As they are going down the river, their boat gets caught in a dangerous current and capsizes. When they surface they find themselves in a prehistoric valley where they meet caveman Gorak, his wife Gara and their two children Lok and Tana. The two families soon become friends and Gorak and his family help the Butlers in their many attempts to find a way to return home while trying to survive in the valley.