This book opens with a prologue that explains the history of a magic tree house that 8-year-old Jack and his younger sister, Annie, have found in the woods. After entering the tree house and looking at the books, the children were transported through time to visit dinosaurs, Egyptian tombs, pirates, ninjas and an Amazon rainforest. During one of their visits, they met an enchantress named Morgan Le Fay, who asked the children to free her from a magic spell by finding four items. So far they have found a moonstone and a mango. A mouse named Peanut has helped them find their way. Then the book begins.
After a swimming lesson, Jack and Annie climb into the tree house and view the two items they have found so far. They realize both items begin with the letter M, which is the first letter of Morgan's first name. They decide that this is a clue that will help them find the remaining two items. Annie spots a book opened to a picture of rocks and snow. She points to it and wishes they could go there. Jack reminds her that they're wearing only bathing suits, but his warning comes too late. The two are transported to a grove of tall trees on a cold white plain.
Annie slips Peanut into the pocket of her backpack, and the children wrap towels around them to keep from shivering. When Jack says he wishes they could go back to Pennsylvania, Annie reminds him that they won't be able to return home until they have completed their mission. They look at the cover of the book Annie is holding and see that it's titled Life in the Ice Age. The book tells about people called Cro-Magnons who lived under cliffs and carried spears to help them hunt animals.
Jack and Annie climb down the rope ladder, and Annie wears her swimming goggles to keep the wind out of her eyes. They walk across the snow toward a cliff to see if they can find a cave where they will be warmer. The cave is warm inside and filled with what Annie thinks are sticks. Jack reads in the book that during the Ice Age, giant bears lived in caves filled with the bones of their ancestors. The children realize that they are in one of these caves. When they hear a loud snoring sound, Jack urges his sister to run as fast as she can to escape the sleeping bear.
They race across the snow toward another cave that is glowing with a soft light. Inside the cave, a fire is burning. There are animal skins and weapons against one wall. The children suppose that a Cro-Magnon family lives there. In the book, Jack reads that the ice-age people carved knives and axes from stone and made musical instruments from bones. He copies some of this information in his notebook. While he is busy writing, Annie finds animal-skin coats for the two of them to wear. Jack reads in the book that the Cro-Magnon women scraped skins with rocks and sewed the skins together with needles made from bones. The children leave their towels and swim goggles in the cave as a present to the ice-age people in exchange for borrowing the coats.
Using one of the stone lamps in the cave, the children go exploring. Before Jack can warn his sister to stay by his side, Annie disappears into a narrow tunnel. He calls for her to come back, but she insists that she's found something worth seeing. The cave wall is covered with drawings of prehistoric beasts. Jack's book explains that the cave paintings were created more than 25,000 years ago. The ice-age people may have thought that painting pictures of animals gave them special powers to be better hunters.
One painting looks like an animal with an owl face, reindeer antlers and human arms and legs. The animal is holding a flute. Jack reads that this creature was called "Master of the Animals" and was a sorcerer. The owl mask helped the creature see well, and the reindeer antlers helped it run fast. Jack has a feeling that they should look for this creature to get their next clue. They return their lamps to the family's cave and as Jack puts the book in his backpack, Annie notices that Peanut has disappeared. Across the snow are tiny tracks leading out of the cave and onto the great plain.
The children follow the mouse footprints as far as they can, but the wind begins to blow, making the prints invisible. In the distance, Annie sees a giant tiger on a cliff, which Jack identifies as a sabertooth. Realizing they are in danger, they run back to the tree house. Suddenly they fall through the snow into a deep hole. Jack explains that they've tumbled into a trap created by the Cro-Magnons long ago. Branches must have covered the hole, and snow covered the branches, making them invisible. They can hear the sabertooth tiger roaring in the distance.
Just when they think their luck has run out, the children look up to see the "Master of the Animals" staring down at them. Peanut is by his side. Annie asks him to help them out of the trap, and the sorcerer throws them a rope. Annie holds onto it and climbs up the side of the pit. Then she pitches the rope to her brother, who can feel something pulling him as he climbs. When he surfaces, he sees his sister sitting on top of a wooly mammoth. Peanut is perched on the animal's head. Annie explains that the wooly mammoth helped the sorcerer pull them out of the pit. She names the animal Lulu.
Annie realizes that the word "mammoth" starts with the letter M and wonders whether Lulu is the third item that Morgan needs. Jack says he doesn't think so. Lulu kneels down like an elephant so that Jack can climb on. The sorcerer presents them with a small flute made from a mammoth bone. Jack tells Annie that this must be what they have come for and asks the sorcerer whether he knows Morgan. The sorcerer says nothing but unties Lulu and whispers into the animal's ear. Soon they are on their way to the tree house, and the sorcerer has disappeared.
As they ride toward the tree house, the children see elk, reindeer, bison and rhino. They enjoy the prehistoric parade until the sabertooth emerges, scaring the other animals away. Lulu charges toward the tree house, but the tiger stands in its way. Annie suggests that Jack play the flute that the sorcerer has given him. Jack is skeptical but blows into the flute anyway. The tiger freezes in its tracks and finally slinks away.
Lulu allows the children to climb off her back and scale the rope ladder to the tree house. Peanut scampers up, too. They thank the wooly mammoth for helping them and drop their animal skin coats on the ground before climbing into the tree house. On the floor they see the Pennsylvania book, a sign that they have successfully completed their mission. Annie points to the picture of Frog Creek, where they live, and the tree house begins to spin them back toward home.
Annie playfully asks Peanut how he found the sorcerer, but the mouse just squeaks in response. The children review the three items they've found and wonder when and where they will discover the final item that will release Morgan from her spell. Then they head home, happy that they don't have to hunt for their dinner.