This children's chapter book is an installment in the Magic Tree House series. The prologue recaps the series premise: Eight-year-old Jack and his seven-year-old sister, Annie, discovered a magic tree house in the woods of Frog Creek, Pennsylvania, that belongs to Morgan le Fay, a magical librarian from the time of King Arthur. The tree house can transport them to any place pictured in the books it contains. In earlier adventures, Jack and Annie solved four ancient riddles and earned the title of Master Librarians, becoming special library helpers tasked with saving stories from ancient libraries. They received secret library cards marked with the letters M and L. Having already recovered a scroll from ancient Rome and a bamboo book from ancient China, they are about to set out on their third mission.
Jack wakes before dawn, remembering Morgan's warning that this trip to ancient Ireland, over a thousand years in the past, would be dangerous because Vikings raided the coasts. He and Annie walk through the misty Frog Creek woods to the tree house, where Morgan waits. She gives Jack a piece of paper bearing a title in Latin, "Serpens Magna," and explains that during the Dark Ages, the difficult period following the fall of the Roman Empire, educated people in Europe wrote in Latin. She hands them a research book titled
Ireland Long Ago and reminds them of an important rule: In their "darkest hour," only the ancient story they seek can save them, but if they call for help too soon, it will not come. She tells them to show their library cards to the wisest person they meet. Annie points to the book's cover, wishes to go there, and the tree house spins and transports them.
Jack and Annie arrive on a cliff above the ocean on a misty island off the coast of Ireland, dressed in period clothing. Jack reads from the research book that during the early Middle Ages, brave Irish monks helped keep Western civilization alive by copying ancient writings by hand. Steep stone steps are cut into the cliff face, but the steps are slippery in the dark. A rope drops down from above, and they use it to climb.
At the top, Brother Patrick, a cheerful monk in a brown robe, greets them, joking that Jack looks like another invader. Annie claims they arrived by boat. Brother Patrick tells them they are on an island off the Irish coast and warns about Viking raiders, whose ships have prows carved in the shape of serpent heads. He reassures them that Vikings cannot land safely before daylight. Jack and Annie show him their library cards, and the monk recognizes their significance, bowing to welcome them. When Jack shows Brother Patrick the Latin title, the monk gives a mysterious look but changes the subject, inviting them to visit the monastery.
Brother Patrick leads them through the walled monastery, which contains a small church, a vegetable garden, and six beehive-shaped stone huts used for sleeping and daily tasks. The largest hut serves as the library, glowing with candlelight. Inside, monks read, play chess, and write and paint in books. Annie declares they have found "civilization," and Brother Patrick agrees, calling the monastery the place where civilization hides on their lonely island. He explains that the monks record Christian stories as well as old Irish myths gathered from storytellers who sing tales from a time when people believed in magic.
Brother Patrick introduces them to Brother Michael, an elderly monk who has spent his entire life creating a single hand-decorated book. The book has a jeweled cover and pages filled with ornate writing and delicate paintings. Brother Michael explains he writes on sheepskin with goose quill pens and makes paints from earth and plants. When Jack shows the old monk the Latin title, Brother Michael turns to the very page he has been painting. The words "Serpens Magna" appear at the top.
Jack and Annie celebrate finding the story, but Brother Patrick explains that Brother Michael has not finished the page, so they cannot take it yet. Unsure whether they can return, the children decide they must try to leave. The church bell rings for sunrise prayers, and Brother Patrick leads them outside to watch the sun rise over the ocean. He recites, "Shine, O light of the sun, on this day filled with wonder" (36), and tells them such sights inspire the monks' work. He directs them to the cliff path and urges Jack to go while the weather holds, warning that everything can change in an instant.
As Jack and Annie head toward the cliff steps, Jack spots three Viking ships with bright sails and serpent prows on the horizon, heading straight for the island. He races back to warn the monks. Thick fog rolls in as they reach the monastery and ring the church bell. Brother Patrick orders the monks to gather the books and hide in a secret cave on the other side of the island. He tells Jack and Annie to avoid the cliff steps and instead take a path between two large rocks at the cliff's edge. Brother Michael hobbles out and presses his jeweled book into Jack's hands, saying it is better the world should have some of it than none at all. Jack places the book in his leather bag, and they race away.
They descend through the fog to the shore, where the three Viking ships sit deserted, tied to rocks and bobbing in shallow waves. Nearby, Viking warriors study the cliff above, planning to climb it. Annie suggests hiding inside one of the ships. They choose the smallest vessel, but when Jack pulls Annie aboard, the anchoring rope slips off its rock, and the ship drifts out to sea.
Fog obscures the island completely. They find oars and try to row, but the wind picks up and the current rips the oars from their hands. The sky turns black, rain pours down, and a huge wave crashes over the side of the ship.
Annie declares it is their darkest hour. Jack pulls out Brother Michael's jeweled book, holds it up, and cries out for the story to save them. A giant sea serpent rises from the waves, its neck as tall as a two-story building, covered in green scales. Jack is terrified, but Annie calls the serpent beautiful and begs it to push them to shore. The serpent presses its neck against the back of the ship and drives it forward over the waves. The wind calms, the clouds lift, and sunlight returns. With one final push, the serpent sends the ship onto a sandbar near the rocky shore where the tree house sits. Its scales glitter pink and green in the sunlight before it dives beneath the sea.
Jack and Annie climb onto the wet sand, but Annie spots two Vikings on the cliff above. They scramble up the rocks to the tree house. Jack points to a picture of the Frog Creek woods in one of the tree house books and wishes to go home. The tree house spins and carries them back to Pennsylvania.
Morgan waits in the tree house. Jack gives her Brother Michael's jeweled book, and she calls it a great work of art. He explains the story is incomplete because Brother Michael did not finish it. Morgan says sadly that they have only bits and pieces of many wonderful old stories. She reveals that "Serpens Magna" is an ancient Irish tale about a great serpent named Sarph. Annie exclaims that Sarph saved them by pushing their ship over the stormy waves. Morgan reflects that sometimes monsters can be heroes and that Vikings, once they settled down, added much to civilization. She thanks Jack and Annie for their courage, calling them heroes too, and tells them to return in two weeks for their fourth and final mission: finding a lost book in ancient Greece. Jack and Annie walk home as the sun rises. On the porch, Jack watches the sunrise and quietly repeats Brother Patrick's words, "Shine, O light of the sun, on this day filled with wonder" (71), before slipping inside.