After a prologue that imitates an Eastern European black-and-white short film called La llave, the film unfolds in a retrospective frame. P. Tinto is obsessed with building a big family and secretly enlists a blind girl, Olivia, into his grand life project. Despite their good intentions, the couple struggles to have kids as they fail to understand the sexual innuendo of the adults around them, spending years trying to coax life by tugging suspenders and singing “tralari, tralari.”
Several years pass, and the now-elder couple pray for a miracle. That very night, two Martians crash at their doorstep, landing with a misread assumption that they are children due to their small stature. Believing they’ve found new kids to raise, P. Tinto adopts them, even as the Martians protest their adulthood. In the meantime, a nationalist contractor named Usillos becomes involved when a ride to the house goes awry and his car breaks down right outside the Tinto residence. Usillos takes on a role that will intertwine with the growing fantasy around the unexpected family.
Mistaken as an African orphan, Pancho Jose is taken in by the couple, while Usillos is hired by P. Tinto to rebuild the family library into a new room for Pancho. Usillos agrees to the job after learning that a hidden UFO device lies inside the house. Pancho Jose soon bonds with the Martians after discovering the UFO is also a time machine, and he volunteers to help fix it so he can travel back in time to save his mother from a cruel accident and prevent a sorrowful sequence of events from unfolding.
Yet P. Tinto has his own plans. When he learns that the factory’s biggest client, The Vatican, could pull out of a crucial contract, he tries to lure Pancho into his business. Pancho, pivoting toward a clever idea, suggests a pizza venture, but Tinto’s anger rises and he locks Pancho in the attic, where he discovers the time machine had already suffered a strange malfunction during a prior test run. Undeterred, P. Tinto decides to press ahead with Pancho’s idea, only to meet resistance from the town’s priest, who seeks to recruit any “kid” for the church and thus enlists José Ramón, one of the Martians, in the process. Ramon, inspired by faith, chooses the path of a priest and lets go of the plan to return to Mars.
Meanwhile, Pancho Jose begins to charm Olivia, who has long complained of a pain in her lower stomach. The next morning, she dances with a burst of happiness, only to be struck and run over by a train. The heartbreak fuels Pancho’s resolve, and he decides to rebuild the machine using Usillos’s truck. Usillos, possessing growing knowledge about the UFO, reports it to NASA; the investigators arrive and, amid confusion, take Usillos away, finding him dressed in UFO gadgets with a wildly swollen thumb.
Pancho Jose finally completes the machine and says goodbye to P. Tinto, departing with the other Martian. He’s not alone; a midget friend from the madhouse accompanies them, also revealed to be a Martian hidden in the luggage. They travel back in time to save Pancho’s mother and to reunite her with his past self, triggering a cascade of changes in the time stream. Olivia’s death is undone, and an African orphan is guided toward P. Tinto’s house, now perched on the roof, awaiting a miracle.
As the credits roll, a mid-credits scene reveals the stranded Martian is found on the road and, despite insisting he’s an alien, is once again mistaken for a child and adopted by a large family alongside another African orphan. In the final moments, Olivia is shown running happily through meadows, a quiet testament to the tangled but hopeful journey that unfolded.
