El Casper (Edgar Flores) (aka 'Willy') is a gang member initiating a young boy into his gang, the Mara Salvatrucha. The boy is given the nickname "Smiley" (Kristyan Ferrer) following a violent initiation. Casper is romantically involved with a girl, Martha Marlene, but keeps the relationship a secret from his gang, fearing for the girl's safety. When she follows him to a gathering of his gang, the leader, Lil' Mago (Tenoch Huerta Mejía), insists on escorting her away in private, despite the protests of Casper. During an attempt to rape Martha Marlene, Li'l Mago accidentally kills her. He conveys this to Casper, telling him that he'll "find another".

Lil' Mago brings Smiley and Casper to La Bombilla, a location along the train tracks where illegal immigrants stow away on passing trains for travel to the United States. Among the illegal immigrants is a Honduran family the viewer has been previously introduced to, consisting of a teenage girl Sayra (Paulina Gaitán), her father, and her uncle, who are on their way to New Jersey to live with relatives. Lil' Mago, Casper, and Smiley rob the passengers for any money they have until Lil' Mago spots Sayra and attempts to rape her. El Casper, still grief-stricken and possibly seeing parallels with Lil' Mago's treatment of his girlfriend, intervenes, killing Lil' Mago and urging Smiley off the train.

Throughout the train ride, Sayra keeps approaching Casper with curiosity despite her father's advice. Smiley goes back to the gang, telling them what happened. Furious, the new gang leader, El Sol, accuses Smiley of collusion. Smiley timidly protests, begging to be sent to kill Casper to prove his loyalty. Back on the train, Casper, who has smuggled gang members in the past, knows the nuances of the journey, instructing fellow passengers when to get off the train and run around to avoid immigration officers. At one point, it appears that Casper has been accepted by Sayra's family. While they are sleeping, he leaves the train quietly, only to discover Sayra has followed him. The two journey north on a car transport while evading local franchises of Casper's gang.

At an immigrant shelter, Sayra sees a familiar face and asks for news of her father and uncle. She is told that her uncle has been deported and that her father "is no longer with them" (though the English subtitles simplify to "he is dead"). She half-smiles in disbelief and storms away.

At a river crossing where the attendant can only cross with one person at a time, Casper pays their fares with the digital camera containing the cherished pictures of his murdered girlfriend and insists Sayra goes first. Just as she is halfway across, the gang finds Casper. After a desperate chase, Smiley encounters him on the river bank and shoots him.

The dénouement shows Sayra outside a mega mall in the United States, calling the phone number of her dead father's new family. Her uncle is also shown setting off on another attempt to cross the border. Smiley, now accepted by his gang, gets a tattoo commemorating his loyalty.