Twin brothers Zip and Zap find themselves at the summer school known as Hope, which is an institution run by the strict and authoritarian Falconetti (Javier Gutiérrez). This re-education center eliminates all forms of entertainment and leisure, focusing solely on discipline. However, the center's rigid environment takes a turn when Zap gets into trouble for being disruptive and is consequently locked in the notorious "Isolation Room" to contemplate his actions. During this solitary period, he discovers a hidden bag of marbles, which sparks the formation of the "Marble Gang" that includes his brother Zip, Filo, Micro, and Matilda, the latter being the niece of Falconetti.
As a group, they engage in a series of mischievous pranks, including vandalizing the statue of Sebastián Esperanza (Álex Angulo), the revered founder of the school. Each of their escapades is marked by the leave-behind of a single marble, signifying their playful rebellion against Falconetti's stringent rules. When Falconetti attempts to quell their antics, he inadvertently damages the statue, revealing a secret map that supposedly indicates the location of hidden diamonds buried beneath the school grounds.
With a newfound sense of adventure, Zip and Zap cleverly steal the map from Falconetti's office and set off in pursuit of the hidden treasures while being hunted down by Falconetti himself. The Marble Gang eventually stumbles upon a mysterious room filled with toys, and Zap suspects that the "diamonds" mentioned in the map may actually refer to the toy collection once belonging to Sebastián Hope.
When Falconetti discovers the gang, he reveals that the school was originally a playful institution run by Hope before he took over and altered its direction to appease his father. In a twist of events, Falconetti imprisons them in the toy room, but Micro's sharp eye catches sight of a strange machine capable of transforming their collected toys into letters that spell "TOYS." As the gang inserts items such as a wooden mallet (T), steering wheel (O), slingshot (Y), and a duck toy (S), music begins to resonate throughout the school.
The melodious sound originates from the statue of Hope, and the underground chamber where Zip and Zap were confined starts to ascend to the main garden, attracting the attention of all the other students and teachers. Their peers rally behind the Marble Gang, leading to a chaotic but triumphant moment when another teacher takes a stand against Falconetti, locking him away until the authorities arrive.
As the story wraps up, Zap reflects on their summer, stating, > “we never found the diamonds, but it was the best summer we ever had.” The five members of the Marble Gang come together, holding their last marble and tossing it into the sky, revealing that the true treasures were never jewels but rather the unforgettable memories they created throughout this exhilarating summer adventure.
