Summer of the Sea Serpent

In this installment of the Magic Tree House Merlin Missions series, the young siblings Jack and Annie travel through time on quests assigned by Merlin the magician, using a magic tree house in the Frog Creek woods as their portal to distant lands.

On the summer solstice, the longest day of the year, Jack sits on his porch while Annie spots a seagull circling overhead. The ocean is two hours away, so Jack finds the bird strange. Annie suspects it signals that the magic tree house has returned after months away. They follow the gull into the woods and find the tree house in the tallest oak. Inside, they discover a pale blue seashell inscribed with a rhyme in Merlin's handwriting. The rhyme outlines their quest: before nightfall, they must retrieve a Sword of Light or the king of Camelot, the legendary kingdom of Arthurian lore, will meet his doom. The tasks include summoning a Water Knight, passing through a giant spider's cave, swimming with a selkie (a being who is human on land and a seal in the sea) clothed in green, entering a dangerous cove known for deadly storms, diving beneath the Cloak of the Old Gray Ghost, and answering a question with love rather than fear. Annie dreads the spiders, and Jack worries about the ghost, but they hope their friend Teddy, a young sorcerer, will join them. Jack wishes them to the time before Camelot, and the tree house transports them.

They arrive on a sea cliff overlooking a wild, uninhabited coast with three coves below. Finding no sign of Merlin or Teddy, they descend to the nearest beach, leave their shoes on the shore, and Annie calls for the Water Knight. A giant swirl of spray reveals a knight in silver armor astride a creature with a horse's head and front legs but a silvery fish tail. They wade out, climb aboard, and ride swiftly across the cove until the creature rears up and dumps them into the cold water. The Water Knight points toward boulders at the base of a cliff, waves farewell, and vanishes.

They swim to the boulders and find a cave mouth covered by a giant cobweb. Jack hurls a stone through the web to clear a path, and they enter. Deep inside, they reach a round chamber where the rising tide floods in, sweeping them off their feet. They cling to a mossy ledge, unable to retreat through the surging water. Jack spots the Spider Queen on the ceiling: a spider larger than he is, with eight glowing red eyes. Annie panics, but the Spider Queen spins a sturdy web ladder from the ceiling crack down to the ledge and urges them to climb. Jack trusts her and goes first, with Annie following. They squeeze through the crack and drop into the water of the second cove.

Two sleek gray seals speed toward them, nudge them playfully, and guide them to shore. Despite Jack's protests about wasting time, they rest on a warm rock and fall asleep. A friendly voice wakes them: Teddy, the freckled, red-haired young sorcerer from their past adventures. He introduces Kathleen, a girl of about thirteen wearing a green dress woven from sea grass. She is one of twenty selkie sisters who live in a cave in the cliffs. The two seals were actually Kathleen and Teddy, transformed by selkie magic. When Jack mentions the Sword of Light, Kathleen warns that many seekers have entered the cove beyond the Jellyfish Cave, an underwater passage filled with jellyfish, and perished in sudden storms. No selkie has ever dared swim into the Cove of the Stormy Coast.

Kathleen agrees to join them despite the danger. She gives Jack and Annie spare seal skins and speaks a selkie spell. When they dive in, the skins merge with their bodies, granting them flippers and powerful swimming ability. In seal form, they speed through the Jellyfish Cave, where thousands of jellyfish drift harmlessly past their tough skin. They emerge into the milky-green waters of the Cove of the Stormy Coast, eerily silent and calm. They haul themselves onto a small rocky island, and their seal skins slip away, restoring their human forms.

A dense gray fog rolls down from the mountains and blankets the cove. Kathleen identifies it as the Cloak of the Old Gray Ghost, the selkie name for very thick fog, which matches the rhyme and relieves Jack's fear of encountering an actual ghost. Teddy and Kathleen become seals again to search the cove's floor while Jack and Annie stay human so they can grasp the sword. After tense waiting, they hear seal barks, dive toward the sound, and find the two seals circling over a golden sword handle protruding from the sandy bottom. Together, Jack and Annie pull the blade free and carry it to the surface.

As they reach the rocky island, the water erupts. The green hills ringing the cove are not hills but the coils of a giant sea serpent. The monster raises its head, displaying glistening scales, burning yellow eyes, and a purple forked tongue. It demands to know who they are and why they are stealing the sword. Annie shouts that they are on a mission for Merlin, but the serpent grows angrier. It declares they must answer the question of the sword to prove their worth and coils around the island, threatening to crush them. The serpent demands: What is the purpose of the sword? Jack's first guesses, defeating or slaying enemies, feel wrong. Staring at the glowing blade against the sunset sky, he feels an unexpected calm and recalls the rhyme's instruction to answer with love, not fear. Jack declares that the sword should not harm anyone; it should take away fear, and without fear, people stop fighting. The purpose of the sword is peace. The serpent touches its tongue to the blade, then slowly uncoils and sinks beneath the water.

Teddy begins a magical rhyme calling on the sword's light, and when he stalls, Kathleen finishes it in selkie language. The sword blasts a beam of light that forms a solid bridge from the rocky island to the sea cliff where the tree house waits. Kathleen must return to her sisters, and Teddy will escort her. They promise to meet again. Jack and Annie walk up the luminous bridge carrying the sword, and when they step onto the cliff, it shatters into golden sparks.

Merlin steps from the shadows and explains the quest's backstory. The Ice Wizard of Winter, a villain who commands winter storms, stole the sword long ago from the Lady of the Lake, the sword's enchantress and guardian. The Lady had placed a spell making the sword powerful only in worthy mortal hands, so the wizard could not use it but refused to return it. The sea serpent, a servant of the Lady, guarded the sword and tested seekers with its question. The summer solstice was chosen because the wizard's powers are weakest that day. Merlin reveals the sword is Excalibur. He will return it to the Lady of the Lake on the Isle of Avalon, and someday, after Arthur becomes king, she will give it to him.

Jack and Annie climb to the tree house. From the window, they watch Merlin wave farewell by raising his hand and spreading his fingers, a gesture that stirs something in Jack's memory. Jack wishes them home using the seashell, and the tree house returns them to Frog Creek at noon, no time having passed. Walking barefoot through the woods, having left their shoes on the distant beach, Jack realizes that Merlin himself was the Water Knight: The farewell gesture was the same one the knight used in the cove. At home, they find their sneakers on the porch, filled with fine white sand and tiny silver pebbles, and a seagull screeches overhead, a trace of leftover magic.