Santiago, born with one underdeveloped lung, dies in bed at age eleven with his parents beside him.

Magos cuts open Santiago's body while alone with him and removes a piece of his single lung, placing it in a jar.

After weeks of painful estrangement from Joseph, Magos leaves the New York estate and returns to Mexico City, bringing the lung jar with her.

Jackie warns Magos against feeding the lung piece, but Magos secretly spoons chicken broth into the jar that same night.

The lung creature escapes from Magos's room at night and sinks its teeth into Lucía's dog Almendra's leg.

Magos convinces Joseph to travel to Mexico City and reveals the growing creature hidden in her closet.

The creature escapes during an argument and attacks Lucía in her bedroom; Joseph strikes it with a hammer despite Magos's protests.

Believing the creature dead, Magos sobs for the first time since Santiago's death and flees the house carrying its body.

Jackie reports the creature has been spotted alive, and Magos leads Lena and Jackie on a search that finds it in a park.

When Monstrilio first sees Joseph at Lena's apartment, the creature speaks for the first time, calling him 'Papi.'

Magos persuades Lena to surgically remove Monstrilio's arm-tail appendage, believing it is preventing him from evolving into a human boy.

After the amputation and a prolonged depression, Monstrilio sheds his monstrous outer form and slowly grows into a boy-shaped being.

Magos performs her art piece Son on stage, sobbing openly while clutching the pajamas that Santiago died in.

M slips away after Magos's performance and eats his neighbor and the neighbor's cat in the apartment next door.

M wakes each family member to say farewell, folds his clothes beneath the dogwood tree, and walks into the dark forest.

When eleven-year-old Santiago dies, his mother Magos cuts out a piece of his lung and secretly begins feeding it, growing it into a furry carnivorous monster. Despite horror and conflict within the family, Magos nurtures the creature until it speaks and is welcomed into family life as Monstrilio. Believing the creature's arm-tail is preventing it from becoming human, Magos pushes for surgery, and Monstrilio eventually transforms into a boy-shaped being known as M. Years later, Magos publicly mourns Santiago through a performance art piece, after which M's inability to control his appetite leads to a violent attack on a neighbor. Facing the impossibility of living as a human, M finally accepts his true nature, bids his family farewell, and steps into the wild.

The headline events driving Santiago's death, Monstrilio's creation, and M's final transformation.

Santiago's body is cremated and his ashes are scattered at the base of the dogwood tree he used to sit under.

In the weeks after Santiago's death, Magos silently follows Joseph around the house as he grieves openly, driving a wedge between them.

Lena meets Magos at the Mexico City airport and drives her to Lucía's family home.

Jackie finds the jar of lung while helping Magos unpack and tells a cautionary story about a woman who fed a dead girl's heart.

Magos retrieves the creature from a park where it was found near dead cats and brings it back to Lena's apartment over Lena's objections.

Monstrilio wins Joseph over quickly, his vocabulary expands to include 'Mami' and 'Lena,' and the family moves back into their la Roma house.

When Lena forgets to feed Monstrilio one evening, she returns to find the patio surrounded by the carcasses of neighborhood pets.

Lena secretly smuggles a sedated Monstrilio out of the house and into a hospital for a covert MRI scan.

After the arm-tail amputation, Monstrilio sinks into a deep depression and Lena stops visiting the house, wracked with guilt.

Lena accepts a consulting position at a New York hospital and leaves Mexico City for a fresh start.

Peter proposes to Joseph at an exclusive restaurant after a surprise seven-course meal on a romantically lit patio.

Joseph visits his Uncle Luke in Cobble Hill alone to share the news of his engagement to Peter.

During their first sexual encounter, M cannot stop himself from biting Thomas, who is hurt but eventually forgives him.

Romy helps M set up dating app profiles, and M goes on dates with men who allow him to bite them.

The family gathers at the Upstate New York estate for Peter and Joseph's wedding, where M falls asleep on the couch between Magos and Joseph.

In the immediate aftermath of Santiago's death, his ashes are scattered and his parents begin to drift apart as their grief pulls them in different directions. Magos returns to Mexico City where Lena tries to support her, and key relationships begin to form around the growing lung creature. As Monstrilio joins family life back in la Roma, his caregivers struggle to balance love and practicality, leading to difficult and guilt-ridden decisions about his care. The family slowly scatters geographically, with Lena moving to New York, Joseph beginning a new relationship with Peter, and Magos eventually taking M to Berlin. They are finally brought together again for a wedding at the house where Santiago first died, bookending a story defined by shared loss.

The secondary moments that shaped how grief, love, and family bonds were tested.

Joseph walks in on Magos tidying Santiago's body after the death and accuses her of destroying their son.

The morning after secretly feeding the lung piece, Magos is shocked to find it has absorbed all the broth overnight.

At the gathering after Santiago's memorial mass, the lung escapes, bites Magos's finger, and Lena sees the creature for the first time.

Magos sneaks downstairs to lie beside Joseph, who admits he stopped missing her and confesses the anger that kept him alive.

Lena discovers Santiago's old notebook filled with monster drawings, including one creature that looks exactly like Monstrilio.

Lena begins to feel a sense of kinship with the lung creature, and the family starts calling it Monstrilio.

Monstrilio gives Joseph a deep cut requiring eleven stitches, and Magos seizes the moment to push for the arm-tail amputation surgery.

Lena refuses to perform the amputation but agrees to sedate Monstrilio for tests, and the creature appears to die from the injection.

Joseph and Lena secretly plan to euthanize Monstrilio, but Magos discovers their plan and expels both of them from the house.

Before leaving Mexico City, Lena visits Lucía, who tells her that Monstrilio is now walking upright and transforming into a boy.

When Joseph tells Lena about his engagement, she questions his hesitation to tell Magos and accuses him of being in 'Magos's camp.'

Outside the Holocaust Memorial in Berlin, M quietly tells Joseph he is glad his father is happy with Peter.

During sex with Peter, Joseph is suddenly overcome with an urge to bite him and cannot stop himself from sinking in his teeth.

M receives a message from a man named Sam who asks to be eaten, believing it will transport him to another realm.

M shows Lena that his arm-tail stump is visibly regrowing, and she asks quietly if becoming Monstrilio again is what he truly wants.

From the moment Joseph discovers Magos mutilating Santiago's body, the characters' responses to grief begin to diverge in ways that will define all of their relationships. Key revelations—the lung absorbing broth overnight, Lena seeing the creature for the first time, and the discovery of Santiago's monster-filled notebook—mark the creature's emergence as both a mystery and a source of family conflict. The decision to amputate Monstrilio's arm-tail, his apparent death from sedation, and the interrupted euthanasia plot represent pivotal moments of fracture and moral compromise for Lena and Joseph. As M grows into a young man, new turning points emerge: M's violent biting urge during intimacy, Joseph discovering his own parallel impulse, and M's receipt of Sam's disturbing request. The final revelation that M's monstrous nature cannot be suppressed, signaled by his regrowing arm-tail and a quiet conversation with Lena, sets the stage for his departure from human life.

The key discoveries, decisions, and emotional shifts that changed every character.