Jack and Annie have become Master Librarians and have received secret library cards from Morgan le Fay, an enchantress. Before they leave for a family vacation, Annie suggests they check the woods to see if the tree house is visible again. Jack agrees. Annie wonders if a nightmare she had about fires burning and ash darkening the sky is a warning from Morgan.
Morgan is waiting for the children in the magic tree house and tasks them with retrieving a book lost from ancient Rome. She gives them the book Life in Roman Times, tells them that only the ancient story can help them in the darkest hour and gives them togas to wear.
The tree house lands in an olive grove outside Pompeii. Annie thinks something is wrong and wants to return home, but Jack insists they complete their mission. They walk into town, noting how similar daily life in ancient Rome is to life back home. As the children enter the city’s forum, they see a soothsayer shouting that the end is near.
As Jack and Annie look for a library, they pass the public baths and a temple. During their search, they see gladiators and the amphitheater where they fight. When Annie sees the soothsayer again, she approaches the woman. The soothsayer tells the children that the birds are gone, the streams have dried up, the sea boils and the ground shakes. She claims all these signs mean the end is near. Jack and Annie ask about the library, and the soothsayer directs them to the home of a man named Brutus.
Brutus and his family have left the house empty, but Jack and Annie are able to find a room full of scrolls — the library. After searching, they find the scroll Morgan sent them to collect. Jack checks the book Morgan gave them to see if it contains any information about the scroll. Instead, he learns that Mount Vesuvius will erupt that day. Using a sundial, the children discover that they’ve run out of time. The volcano is about to erupt.
The ground begins to shake, and soon the sky has gone dark. Jack and Annie run back toward the tree house, using pillows to protect their heads from falling rocks. They need to cross a stream, but the bridge has collapsed, and they get stuck in the dry streambed full of ash and pumice.
Remembering what Morgan said, Annie grabs the scroll and asks for help. A large man dressed as a gladiator appears and carries them to the other side of the stream. Jack and Annie make it to the tree house and travel home.
Morgan is glad to see the children return safely. She thanks them for retrieving the scroll and reveals she has another mission for them in ancient China. She tells them the next book contains the story of Hercules. Annie and Jack believe the man who saved them by the stream was Hercules. They return home just in time to leave for their family’s vacation.