The novel follows 12-year-old P.T. Wilkie, a born storyteller who lives with his mother and grandfather at the Wonderland Motel on St. Pete Beach, Florida. The motel was opened by P.T.'s grandfather, Walt Wilkie, in 1970 as a small resort and amusement park called Walt Wilkie's Wonder World, but it has long since declined into a half-empty motel decorated with oversized fiberglass statues, including a dinosaur, a rocket ship, a jackalope, and a giant mouse. P.T.'s mother, Wanda Wilkie, manages the property and worries constantly about money, while Grandpa tinkers with attractions and reminisces about the old days. P.T.'s father left before P.T. was born, and though P.T. masks the absence with invented cover stories, telling classmates his dad is an "undercover astronaut," the loss is real.
P.T.'s world is upended when a banker named Mr. Pompano informs Wanda that Grandpa secretly took out a $100,000 loan with a balloon payment, meaning the full sum is due in 30 days or the bank will seize the property. While P.T. distracts Grandpa from the meeting, Grandpa tells his favorite story about Sheila, a glamorous woman in a red polka-dot scarf who visited Wonder World in 1973, rode the park's train every day for a week, and then left for the nearby IHOP, never to return. P.T. overhears the banker tell Wanda that the property "could be huge," hinting at outside plans for the land.
With Spring Break beginning, Wanda cuts costs by firing housekeepers, eliminating amenities, and making P.T. the new pool attendant. P.T. meets Gloria Ortega, a guest his age whose father, Manuel "Manny" Ortega, is a sportscaster newly hired at local TV station WTSP. Gloria is a veteran of Junior Achievement, a youth business-education program, with sharp instincts but no capital. One evening, P.T. watches a true crime show about the 1973 Miami Palm Tree Hotel heist, in which brothers Sidney and Stanley Sneemer stole $5 million worth of diamonds. Both were imprisoned, but the jewels were never recovered.
Desperate to save the motel, P.T. and Grandpa launch a poolside act: P.T. hides a walkie-talkie inside the frog waterslide, and Grandpa voices "Freddy the Frog" from his workshop, bantering with guests through corny jokes. Gloria offers to become P.T.'s business partner, rebranding the ice-cream machine as "Freddy the Frog's Frozen Green Pond Scum," launching "Croaky Karaoke" sing-alongs for college students, and selling rubber frog souvenirs. The shows earn nearly $3,000 in a single day with 10 new rooms booked.
To build momentum, P.T. and Gloria plan a pirate treasure hunt. They bury a chest of cheap toys and gold-foil coins on the beach, and Gloria persuades her father to interview P.T. live on the WTSP morning news. P.T. invents the legend of "Stinky Beard, the smelliest pirate on the seven seas," and hundreds of people show up. The motel's No Vacancy sign lights up for the first time in decades. Among the new guests is an elderly man named Bob Jones, who says a "lady friend" once sent him a postcard from Wonder World.
The success creates problems. Local businesses complain that the hunt diverted their customers, and P.T.'s history teacher, Mr. Frumpkes, whose mother lives nearby, files a police complaint. Officers tell Wanda to limit future promotions to motel property. P.T. and Gloria also overhear Mr. Pompano and a developer named Arnold discussing plans to demolish the Wonderland and build a luxury condo complex once the loan defaults. Gloria confronts them with intimidating financial jargon, rattling Arnold visibly.
A second elderly man, Johnny Jones, arrives claiming to be Bob's brother and says their sister is Sheila, the woman from Grandpa's story. P.T. hides behind the frog slide and overhears the brothers arguing, calling each other "Stanley" and "Sid." They discuss how Sheila double-crossed them, how they each spent over 30 years in prison, and how they believe she hid "the ice," slang for diamonds, on the motel grounds before her arrest at the IHOP. P.T. connects them to the Sneemer brothers from the crime show and realizes the Wonderland may hold the stolen jewels, now worth approximately $27 million.
P.T. borrows a master key from Clara, the longtime housekeeper, and he and Gloria search both brothers' rooms. They find postcards from Sheila. Stanley's card taunts him as a "big, big man" who is "all empty inside," while Sidney's tells him to "follow my tracks." Both clues point to Smilin' Sam, a 25-foot-tall hollow fiberglass statue on the property. When the Sneemers catch them, P.T. improvises a cover story and claims Grandpa found a map Sheila left behind.
P.T. gives the brothers a copy of the old treasure hunt map, which leads them to dig at night behind Mr. Frumpkes's mother's fence. They trigger motion detectors and alarms and are arrested. With the Sneemers in jail, P.T. and Gloria cut open Smilin' Sam with power tools but find nothing inside. Wanda catches them, and Grandpa points out a plaque reading "Installed 1976," three years after Sheila's visit. The jewels cannot be hidden there.
The next morning, a private investigator named Big Irv arrives. He works for the Amalgamated Insurance Company, which offers a $150,000 reward for the stolen jewels. Big Irv concludes the jewelry is inside Smilin' Sam. Since P.T. already knows the statue was installed too late to contain the loot, he stays silent and lets Big Irv reach his own wrong conclusion. Wanda sells the damaged statue to Big Irv for $6,500.
To raise more money, Wanda and Grandpa sell the motel's remaining attractions. The Wonder World Express train is rented to Wild Cat Safariland, a big-cat sanctuary in Tampa. Even after all the sales, the family is still $40,000 short. Then Mr. Ortega notices a sign on the train's caboose: "Follow Me to Happy Town!" P.T. connects this to Sheila's postcard clues and realizes the jewels are hidden on the train, explaining why Sheila rode it every day.
Grandpa drives P.T. and Gloria to Wild Cat Safariland after dark, and they search the train. In the caboose, P.T. notices four red lanterns on the corners. Inside the last one, he discovers a rusty Band-Aid tin containing a Miami Palm Tree Hotel envelope stuffed with diamonds, emeralds, and rubies.
Big Irv, who attached GPS trackers to the family's vehicles, arrives and demands the jewels at gunpoint, admitting he plans to keep them rather than return them to the insurance company. P.T. has already slipped the tin into his back pocket, leaving only the empty envelope visible. He provokes Big Irv with a rambling story until the detective fires his gun at the ceiling in frustration. The gunshot startles a tiger in a nearby cage, which charges after Big Irv. The detective flees with only the empty envelope, and P.T. calls 911 and contacts the insurance company.
The family turns over the jewelry and collects the full $150,000 reward, paying off the loan a week early and thwarting the banker and the developer. With leftover funds, they buy back the sold statues and have them reinstalled. The damaged Smilin' Sam is repainted as Ponce de León, complete with a conquistador helmet. Gloria enrolls at P.T.'s school, and the Ortegas continue living at the Wonderland. Freddy the Frog resumes weekend shows, and with roughly $30,000 remaining, the family debates how to invest in the motel's future. P.T. closes by addressing his absent father, affirming his love for life at the Wonderland: "We're having a wonderful time at the Wonderland. Wish you were here."