Evil sorcery. A head in a box. Jellyfish boys. Nasty pizza. These are just some of the threats faced by Thatcher Hill and his friends, superhero-in-training Trudy McGee, and Princess Shoal, heir to the throne of sunken Atlantis.
This wasn’t the summer vacation Thatcher was expecting. He was supposed to be traveling through Asia with his parents. Instead, he finds himself stuck in the small beach town of Las Huesas, California, helping his great-uncle Griswald run a seaside freak museum. His companions are a headless mummy, the FeeJee Mermaid, and the What-Is-It???, a sealed box that no doubt contains some disgusting thing probably better left unseen.
A week into his stay in Las Huesas, Thatcher awakens to the sounds of someone breaking into the museum. As usual, Griswald is off drinking with his salty old cronies, so it’s up to Thatcher to brave rocks and surf and dangerous crustaceans in pursuit of the girl-thief who nabs the What-Is-It??? But the girl is too fast, and Thatcher loses her on the moonlit beach.
The next morning, Thatcher meets Trudy McGee, superhero-detective-in-training, who’s been keeping track of the strange doings in town. She’s an odd girl who uses big words and reminds Thatcher of Batman. So, he likes her. Together, they track down the thief, a girl named Shoal, who happens to be the crown princess of lost Atlantis. The survivors from the sunken city suffer under a witch’s curse. For most of the year, they float like flotsam across the ocean. But in summer, they wash up on the shores of Las Huesas and are compelled to work the hotdog stands and tourist shops and midway games. Shoal stole the What-Is-It??? because it contains the still-living head of Skalla, the witch.
Skalla commands a cadre of human/sea-creature hybrids who do her bidding. Two of these, a pair of jellyfish boys, attack Thatcher, Trudy, and Shoal and make off with Skalla’s head. Thatcher and his new allies manage to get the head back, but not before the witch casts the Flotsam curse on Thatcher and Trudy. Then the witch falls silent, retreating into a power-preserving hibernation. The kids must find a way to reverse Skalla’s spell by the end of summer. Definitely not the vacation Thatcher was hoping for.
Things get worse when Shoal is swallowed by a giant fish of Skalla’s making. Just before disappearing down the fish’s gullet, Shoal urges Thatcher and Trudy to seek her father in the Atlanteans’ summer palace, a decrepit mansion on a hill outside of town. There, Thatcher and Trudy encounter Shoal’s family, including King Coriolis, Shoal’s father. Coriolis charges them with finding the Book of Keepers, a tome containing a spell that can force Skalla to speak and spill her secrets. Thatcher promises the king he’ll locate the book, a task that becomes absolutely crucial once the king’s sorcerer reveals that the Atlanteans will return to the Drowning Sleep not at the end of summer, but in a mere three weeks. And more, Skalla is planning a bigger spell of some kind, something catastrophic. The king and the rest of Shoal’s family are kidnapped by a band of raiding lobster men, more of Skalla’s creatures, and Thatcher and Trudy barely avoid capture themselves.
They find the book in the possession of the jellyfish boys, whom Thatcher learns were once just a couple of regular kids, not so unlike him, until they fell under the witch’s power. They also discover that Uncle Griswald is a nKeeper, a land-dweller devoted to helping the Atlanteans in their long struggle against Skalla. Using the book, Thatcher and Trudy cast the Spell of Compelled Interlocution on the witch. Skalla is forced to reveal that they can recover Shoal with the help of a minor sea god who calls himself the Beachcomber. A harrowing trip through the spooky Tunnel of Love transports Thatcher and Trudy to a lost beach. There, the Beachcomber helps them out by catching the monster fish that ate Shoal, but it’s up to Thatcher to hack his way inside the fish and retrieve her. In the depths of the fish’s belly, he finds himself in a funhouse made of guts, where his own reflections accuse him of being nothing more than a smart-mouthed lightweight, someone who merely uses words as armor. Thatcher’s determination to stand by his friends keeps him focused on the task at hand, and he leads Shoal out of the fish.
Shoal realizes the witch made a mistake by letting her stew inside the fish’s belly, for when Skalla works her magic on a living being, some of her magic and intentions remain inside the creature. So now Shoal knows what Skalla’s plans are: She’s building a new Atlantis in secret that she can rule with absolute power, and she’s going to summon a tidal wave to drown Las Huesas. To do this, she needs to recover the magic she invested in the kidnapped Atlanteans when she cast the Flotsam curse. And King Coriolis’s sorcerer was wrong. Skalla’s not going to make her move in three weeks. She’s going to do it in three days.
After returning to the Las Huesas boardwalk, the three friends are captured by kelp men, minions of the eel sisters, all of whom are former creatures of Skalla. The eel sisters are up to their own no-good. They want what Skalla wants, only they plan to rule the new Atlantis themselves. Their scheme requires access to magic, which they attempt to obtain by bleeding Thatcher, Trudy, and Shoal dry. But using his gift of gab, Thatcher talks the kelp men into turning on the eel sisters and letting them go free.
By deciphering the encoded hieroglyphs the witch uses to cast her spells, the three discover that the Atlanteans are being held at the decommissioned Ferris wheel on the boardwalk. When they arrive there, everything is in place for Skalla to enact her plan: hieroglyphics, a deep pit containing the new Atlantean palace, and the Atlanteans themselves, strung up on the Ferris wheel, ready to be bled of the magic in their blood.
The kids launch a desperate rescue effort, but they can’t overcome Skalla and her creatures, especially not the worst creature of all, a squid the size of a three-story building. But just before being torn asunder by the squid, with a huge tidal wave thundering toward shore, Thatcher realizes that Skalla’s hieroglyphics reveal something about her, something that she didn’t even know herself: the mummy in Griswald’s museum is Skalla’s body.
Thatcher proposes a deal. Back at the museum, Thatcher, Trudy, and Shoal let the witch draw on the magical residue she left in their blood when she cast the Flotsam spell on them. Skalla’s head is once more united with her body, and she is restored to youth and health. In return, Skalla releases the Atlanteans, undoes her curse, and abandons her plan to destroy Las Huesas.
Thatcher goes back home to his parents and school in Phoenix. He’s more confident and less compelled to speak without thinking. But he feels adrift, no longer fully at home in Phoenix. He’s become like flotsam, floating between places. But when he returns to Las Huesas the next summer and finds Trudy and Shoal still there, he knows where his anchors lie.