The story revolves around a mixed-breed dog named Hagen, who becomes a loyal companion to Lili Lili Horváth, a 13-year-old girl. As Lili grapples with her mother's departure and the strict nature of her father—who has been absent from her life for years—Hagen provides her with much-needed comfort. However, the Hungarian government's hefty "mongrel" tax creates tension, as Lili's father refuses to pay it, instead proposing to buy her a purebred dog if she relinquishes Hagen. This moment highlights the growing rift between father and daughter, showcasing their lack of understanding each other's emotions.
Desperate to avoid her father abandoning Hagen while she's away, Lili brings him to her music class. Unfortunately, his presence creates chaos, leading the teacher to threaten her with expulsion unless she takes the dog outside. Frustrated, Lili chooses to leave the class with Hagen, but this decision leads to her father's anger, prompting him to track them down at the music school. In a fit of rage, he drives Hagen far from the city and abandons him, causing the dog to chase after their car in vain.
Left to fend for himself, Hagen embarks on a lonely, perilous journey through the city. He befriends a white dog after being rescued from an irate butcher and passes the night with other stray dogs. However, their peace is disrupted when a dog catcher comes after them. Thanks to the assistance of a homeless man, Hagen narrowly escapes the dog catchers but soon finds himself trapped in a dog-fighting ring, where he is mistreated and forced to fight for survival. In a harrowing turn of events, Hagen manages to escape after slaying his opponent, leading him to be apprehended by animal control and taken to a dog pound.
Meanwhile, Lili is still reeling from the loss of her beloved Hagen. At a party, fueled by her anger and sadness, she drinks too much, resulting in her being picked up by the police and her father being notified. Upon his arrival, Lili's father breaks down, confessing that he regrets forcing Hagen out of their lives and admitting his struggle to connect with Lili as she grows into her teenage years. This emotional confrontation helps them reconcile, and as a sign of his desire to reconnect, he promises to attend Lili's upcoming concert.
In a dramatic twist, Hagen escapes from the pound after an altercation with a staff member, freeing other imprisoned dogs and rallying them under his leadership. Together, they unleash chaos among the humans who have wronged them. Eventually, the pack makes their way to the concert hall where Lili is performing, prompting panic among the attendees. As she spots Hagen, Lili sneaks out and pursues the pack on her bicycle.
The climax unfolds at the slaughterhouse where Lili's father works. Objective for vengeance, Hagen and his newfound allies prepare to confront him. Initially, Hagen growls menacingly at Lili as his army surrounds them. However, her father rushes out, armed but unprepared for the scene. In a touching moment, Lili plays Franz Liszt's "Hungarian Rhapsody No. 2" on her trumpet, soothing Hagen and encouraging the other dogs to lie down in a symbolic act of reconciliation. As Lili positions herself beside Hagen, her father drops his weapon and joins them, marking a profound moment of unity and understanding in their fractured family.
