Shortly before celebrating his eighteenth birthday, Bennie Tetrocini, the son of the abusive yet renowned conductor Carlo Tetrocini, decides to escape his prep school. He finds work as a waiter on a cruise line, but the ship soon breaks down in Buenos Aires, where he unexpectedly reconnects with his half-brother Angelo, who prefers to be called Tetro. Tetro, who left the family years ago to pursue a career in writing, harbors deep-seated resentment and refuses to recognize Bennie as his brother. Meanwhile, Tetro’s girlfriend, Miranda, a compassionate psychiatrist with a past connection to Tetro, welcomes Bennie into their home, despite Tetro's harsh demeanor.
As Bennie's birthday approaches, Tetro reveals that he wrote a play about their family, featuring Carlo and his brother Alfie, who abandoned Tetro in his search for success. Bennie discovers Tetro's script, written in a secret code they both learned during their time at prep school, and begins to transcribe key sections that reference their troubled family dynamics. During this time, Bennie attends a burlesque production of Faust where Tetro is handling the lighting. Tensions rise when Tetro confronts actor and playwright Abelardo, only to be interrupted by a surprise visit from his old mentor, Alone, with whom he has a complicated history.
In the midst of family struggles, Bennie inadvertently violates Tetro's rule about discussing their past when he gives Miranda one of Carlo's CDs. This ignites a fierce argument, revealing Tetro's failures and broken promises regarding bringing Bennie away from their father's grasp. Bennie's journey becomes even more perilous when he tries to save a puppy, resulting in an accident that lands him in the hospital and causes him to miss his ship. With his circumstances changing, he continues to live with Tetro, while Miranda secretly facilitates his writing of the play.
When a crisis strikes the Tetrocini family—Carlo suffers a stroke—they summon both brothers back home, but Tetro refuses. Bennie's determination to understand his family legacy leads him to transcribe parts of Tetro's play, especially the emotional sections regarding Tetro's heartbreak over Naomi, who Tetro believes was stolen from him by Carlo. Bennie's creativity begins to flourish, and he writes an adaptation titled Wander Lust, which earns him a place as a finalist in a festival run by Alone.
As the family travels to the festival, complications ensue, including Bennie's first intimate experience with actresses and Tetro's abrupt disappearance before the performance. However, Tetro ultimately confronts Bennie, confessing that he is in fact his biological father, and that Naomi is his mother—who endures a tragic fate after revealing the family’s complex lineage. This revelation culminates in a tense moment when Tetro asks Bennie to enact the violent ending of his play, but Bennie refuses.
The narrative takes another dark turn when Carlo passes away, leading to a poignant funeral in Buenos Aires. Tetro seizes the opportunity to reclaim his family legacy by taking Carlo's baton and offering it to Alfie. In a moment of rebellion, Bennie, dressed to embody Tetro’s spirit, reveals their truth to Miranda and the family, igniting a destructive fire. Ultimately, he yearns for a reconnection with Tetro, leading to an emotional climax where Tetro accepts Bennie as his son, and they reconcile, affirming that "we're a family."
