The Baxter children are gearing up for Christmas when a bomb explosion rocks their peaceful town. Undeterred they enjoy a Christmas river cruise, but the crew seem to be lacking the Christmas spirit. They find the ship moored up river, and clandestine operations of a criminal gang linked with international terrorists. Before they can inform the police they plummet into a strange but deadly underworld of an unknown civilisation and pre-historic beasts. Can they protect the 21st century world from the onslaught of disease and the ravages of ferocious dinosaurs, while saving their own skins?