Arnold and Gerald spend the summer after their fifth-grade year planning a humanitarian video that could win them a trip to San Lorenzo, a Central American country where his parents were last seen. They hope this project will demonstrate their good intentions, and they first try to document themselves building a makeshift house out of junk for their friend Monkeyman. But as they film, a group of local homeless people arrives, claiming the junk as their property. A chaotic confrontation erupts, the makeshift shelter is destroyed, and Arnold is left heartbroken and demoralized, doubting whether anything he does can really help those around him.
Helga, who has long harbored feelings for Arnold, quietly pieces together a montage of his good deeds from footage she’s kept over the years. When a surprising town-wide screening unfolds, Arnold is stunned to discover that the community has seen his acts in a different light. The moment is brightened by the revelation that the school’s teacher, Mr. Simmons, has announced that Arnold’s video won the competition. The whole class at PS 118 and Helga’s older sister, Olga, celebrate this victory as they prepare to travel to San Lorenzo.
On the plane to the country, Arnold’s beloved pet pig Abner hides away in his backpack, adding a touch of chaos to an already emotional journey. When they land, they are greeted by Eduardo, an old friend of Arnold’s parents, who warmly welcomes them aboard a ship that will ferry them toward the mysteries of the lost city. Eduardo presents Arnold with an amulet said to guide them to the “Green-Eyed People,” the inhabitants of the fabled city that supposedly holds secrets and treasures of great importance.
That night, Helga’s feelings threaten to surface in a confession, but their voyage is interrupted when pirates attack the boat. The group’s trust begins to fracture as they learn a secret Arnold has kept from them, and Gerald, Helga, and the others start to distance themselves from him, unsure of where his loyalties lie or why he kept silent.
When the group finally reaches a base camp, the truth about Eduardo comes to the surface in a dramatic reveal: Eduardo is actually a mercenary known as Lasombra. He and his crew imprison everyone and explain that the “contest” was a trap designed to lure Arnold to San Lorenzo so Lasombra could use him to locate the lost city and its riches. The revelation shatters trust among the classmates, and even the adults, as they confront the deception.
In the midst of despair, Arnold’s friends realize they’ve been misled and rally to help him. Helga and Gerald manage to escape their confinement and free Arnold, using Arnold’s father’s old journal to guide their search for the lost city. Unbeknownst to them, Lasombra anticipated the move and had placed a tracking device on Arnold’s green-eyed necklace, ensuring he could keep tabs on the group no matter where they went. The chase across traps and puzzles tests their wits as they work to outsmart Lasombra and reach safety.
Back home, Abner escapes and heads toward Arnold’s grandparents—Grandpa Phil and Grandma Gertie—who view the distress as a signal that their grandson is in trouble. They connect with Helga’s parents, Miriam and the others, and they all race to San Lorenzo to assist. A rescue mission unfolds, with the group catching a flight to the distant land and joining forces with the locals to push back against Lasombra’s gang.
Arriving in San Lorenzo, the kids discover a city inhabited by children, a result of a sleeping sickness that has left the adult population comatose for nine years. In their search for a cure, they locate a statue rumored to house the Corazón, a treasure that could unlock the remedy. Lasombra corners the group, kidnapping Arnold and stealing the statue’s power for himself. Gerald and Helga pursue, and Arnold is forced to open the statue with the amulet. When Lasombra tries to seize the Corazón inside, the statue unleashes a deadly defense, shooting him with a poisoned dart and sending him plunging over a cliff.
A mysterious figure arrives—the real Eduardo—who explains that the pirate attack at sea was part of his attempt to rescue the group from Lasombra. Lasombra miraculously climbs back up, and a final confrontation ensues. He wrestles with Eduardo, destabilizing the Corazón’s location and ultimately falling to his death after the poison takes its toll.
With Lasombra defeated, the group returns to the city and learns that Arnold’s parents, Miles and Stella, have contracted the sleeping sickness themselves. In a crucial moment, the temple’s magic responds to the group’s need as Helga uses a cherished keepsake—a locket containing a photo of Arnold—to stand in for the lack of the Corazón. The temple releases the cure, reviving the infected children and restoring life in the city.
The reunion is emotional: Arnold is able to embrace his parents again, and the children share a renewed sense of community with their families. Arnold expresses gratitude to Helga for her steadfast loyalty, and the two share a kiss, signaling the depth of Helga’s feelings and the strength of their bond.
Months pass, and life settles back into its familiar rhythm at Arnold’s grandparents’ boarding house, now with Miles and Stella living there as well. On the first day of sixth grade, Arnold bids farewell to his parents and heads off with Gerald, Phoebe, and Helga. The future hints at something more between Arnold and Helga, as their relationship remains complicated in public, with Helga continuing to protect her image even as affection quietly grows between them. The story closes on a hopeful note, balancing friendship, loyalty, and a shy but undeniable connection that promises to endure.
