Walter, a 13-year-old boy, is spending his summer vacation in Rome, still grieving the recent loss of his father who died in a workplace accident. During his aimless wandering along the coast, he stumbles upon what appears to be an abandoned villa, notable for its impressive swimming pool. Curious and adventurous, Walter dives into the pool and narrowly escapes an encounter with a ferocious great white shark that was kept there. The close call leaves him both shaken and fascinated, and he begins returning to the villa daily to observe the mysterious creature.
It is during these visits that Walter meets an older boy named Carlo, who introduces himself as the villa's caretaker. Carlo reveals that the property belongs to a gangster known as Corsaro, which translates to “Privateer,” who uses the villa as a secret hideout. Corsaro keeps the shark as a symbol of dominance and power, boasting that it once devoured an opponent of his years earlier. Intrigued and somewhat awed, Walter forms a bond with Carlo, who allows him to visit whenever he wants, on the condition that he brings food for the shark—a task that leads Walter into a series of petty thefts, selling his father's possessions and stealing money at home, much to the disapproval of his mother, Rita.
Throughout the summer, Walter and Carlo develop a close friendship, sharing stories and adventures. When their funds run low, Carlo devises a plan to rob a butcher's shop to get more money. However, the plan goes awry, forcing Walter to step up and defend Carlo. Threatening the butcher with a gun, Walter manages to secure two carcasses to feed the shark, impressing Carlo. This act of bravery earns Walter entry into Carlo's gang. Soon, Carlo's leader, Tecno, puts Walter to work dealing drugs, further immersing him in the gang's dangerous activities.
As Walter becomes entangled in the gang, his mother, Rita, becomes increasingly worried about his behavior and the choices he is making. Her concern prompts Walter to run away from home and move into the villa with Carlo and the gang. However, their reckless plans escalate. When they discover that the shark is suffering due to captivity, Carlo and Walter dream of freeing it. Unfortunately, they steal money from Tecno to fund their rescue plan, but Walter is falsely blamed for the theft. Tecno punishes Carlo harshly, beating him into submission and expelling both boys from the gang.
Later, Carlo attempts to regain his place in the gang by bringing the others back to see the shark at the villa. The boys begin mistreating the creature, teasing it cruelly. Walter's attempts to intervene are met with brutality from Tecno, who beats him badly. Just as Tecno is about to kill Walter, Corsaro arrives unexpectedly. In a display of authority and strength, Corsaro knocks Tecno to the ground and orders the others to leave his property. Impressed by Walter's courage, the gangster commends him for his bravery.
In the aftermath of these events, Walter returns to his mother, deeply affected by his experiences and newfound understanding of strength and kindness. With Carlo's remorseful help, he is finally able to set the shark free into the sea, symbolizing his growth and the beginning of healing from his loss. This summer becomes a pivotal chapter in Walter's life, teaching him about loyalty, bravery, and compassion amidst a backdrop of danger and adventure.
