Rosario, a resilient single mother, undertakes a perilous journey as she illegally migrates across the US-Mexico border to Los Angeles, leaving behind her young son, Carlitos, in his hometown with his ailing grandmother. Through their weekly phone calls, Rosario paints vivid pictures of her surroundings around the payphone, which Carlitos eagerly listens to. Tragedy strikes on Carlitos’ ninth birthday when he discovers that his grandmother has passed away in her sleep, leading him to learn that his aunt and uncle intend to take custody of him for the money his mother sends home.
Driven by an unwavering determination to reunite with Rosario, Carlitos arranges for coyotes to aid him in crossing the border through El Paso–Juárez. Unfortunately, their vehicle is impounded by the police due to unrelated issues. Stranded in El Paso, Texas, Carlitos inadvertently drops his money as he exits the car. Desperate and alone, he approaches a man for a bus ticket. However, when the man realizes Carlitos has lost his money, he attempts to sell him to a pimp. Luckily, a compassionate local woman intervenes, taking Carlitos under her wing and introducing him to life among other undocumented immigrants.
After a police raid disrupts the situation, Carlitos escapes with Enrique, who is initially reluctant to help him. As they journey together, the pair hitchhiking towards Tucson, Enrique grows frustrated with the persistent Carlitos and angrily tells him to leave. Yet, just as Carlitos starts to walk away, danger looms as a group of men plans to rob him of his backpack. Enrique chooses to protect Carlitos, solidifying their bond.
The duo finds work at a restaurant, and in the process, Carlitos seeks out his estranged father, Oscar Aguilar Pons, who ultimately disappoints him by failing to follow through with promises of support. Undeterred, Enrique decides to take Carlitos to Los Angeles. Their bus ride brings them to the vibrant yet challenging streets of LA. Meanwhile, Rosario grapples with her own hardships, losing her job due to immigration troubles and becoming desperate to locate her son.
Arriving in East LA, Carlitos and Enrique search for a payphone linked to Rosario’s calls, but they only uncover a P.O. box. After a long day filled with frustration, the two take a rest on a bench, unknowingly passing by Rosario. During this time, Doña Carmen informs Rosario about Carlitos’ dangerous crossing and the death of her mother. In a bid for stability, Rosario marries a U.S. citizen for a green card, preparing to return to Mexico in search of her son. As she boards a bus, a fleeting glimpse of a boy at a payphone makes her heart race; she senses that Carlitos is trying to find her as well.
The next morning takes a tense turn when police officers confront Enrique and Carlitos. In a moment of bravery, Enrique throws his coffee at the officers to distract them, urging Carlitos to flee just in time while he is arrested in the process. In a climactic moment, Carlitos finally locates the payphone and sees Rosario waiting for him across the street. She shouts for him not to cross yet—an anxious pause before the crosswalk shifts from the ominous "red hand" to the hopeful “white figure walking" light as the credits roll, hinting at a reunion filled with promise.
