On a rainy day, a school bus suddenly falls into a hole in the ground and lost contact with the outside world. There are four students on the bus the kind-hearted Bai He, the brave Jian Yun, the delicate Xiao Ling and the selfish Jun Shun. Together with the bus driver Uncle Ou, they start a thrilling adventure. In the underground hole, they eat Crossopterygii that lived three hundred million years ago and universally acknowledged to be extinct. They enjoy the bright crystal garden. They touch a giant dragonfly like a helicopter and a large lotus whose leaves are as big as a canopy. There is a mysterious old woman. No one knows whether she is a human or a ghost In the process of facing the difficulties together, the four students lay down resentment and help each other. They pass through all kinds of danger and solves various mysteries. Seven days later, they are rescued one by one except Bai He