The search for five-year-old Milo Irvine led to a web of dark secrets that changed his family forever. The Peacock thriller series All Her Fault reached its conclusion, revealing shocking truths about a kidnapping, a fatal car accident, and the lies told to hold a family together.
The series, based on Andrea Mara’s novel, follows businesswoman Marissa Irvine, played by Sarah Snook, after her son Milo vanishes from a playdate. The mystery brings her closer to another mother, Jenny Kaminski (Dakota Fanning), as they face public judgment and a police investigation led by Detective Alcaras (Michael Peña). The search eventually exposes that the family’s nanny, Carrie Finch (Sophia Lillis), was behind the kidnapping, but her motives were far more personal than anyone imagined.
After Milo is safely returned, Carrie arrives at the Irvine family home armed with a gun. During a tense standoff with Marissa, her husband Peter (Jake Lacy), and others, Carrie’s true identity is revealed. She confesses that her real name is Josephine “Josie” Murphy.
“You don’t know what he’s capable of,” Carrie tells Marissa, urging her to protect Milo from Peter.
Before Carrie can play a phone recording that holds Peter’s secret, he lunges at her, grabs the gun, and shoots her dead, claiming it was an accident. Marissa does not believe him and demands the truth. Peter then confesses to a life-altering crime from six years ago.
He explains that on the night they brought their newborn son home from the hospital, they were in a severe car accident with another vehicle. Peter was the only one conscious and discovered that his and Marissa’s biological son had not survived the crash. Hearing a baby crying from the other car, he found Josephine Murphy unconscious and her infant son alive. Believing Josephine was also dead, Peter made a devastating choice.
“He says that because she was alone in her car, it seemed as though she couldn’t give Milo the life that they had given him,” explains a report of Peter’s reasoning.
Peter switched the babies. He took Josephine’s living son and placed his own deceased child in her car. He then let Marissa, who survived with a shoulder injury from the crash, believe the baby she raised was their biological son. The boy they named Milo was, in fact, Josephine’s child.
Peter’s deception during the car crash was not his only secret. As the series progresses, more lies come to light, damaging his relationships with his own family.
For years, his siblings Lia (Abby Elliott) and Brian (Daniel Monks) believed Lia was responsible for a childhood accident that left Brian disabled. Peter finally admits that he was actually the one who tripped Brian, but he allowed Lia to carry the guilt, which contributed to her struggles with addiction. He also lied to Brian about being a candidate for spinal surgery that could improve his life, preferring to maintain control by being needed.
Marissa discovers another grim truth by listening to the recording on Carrie’s phone. It reveals that Peter met with Carrie’s father, Rob Murphy, who was blackmailing him over the baby switch. Instead of paying the ransom, Peter killed Rob with a lamp. He was also the one who then took Milo from the motel, blindfolded him, placed him in a car trunk, and left him at a police station to be found, carefully covering his own tracks.
Learning that her husband is a kidnapper and a murderer leaves Marissa trapped. She fears that if she goes to the police, Milo could be taken from her, and Peter might harm her. She confides the entire truth to her friend Jenny, stating she is scared of what Peter could do.
Marissa decides to take matters into her own hands. Knowing Peter has a severe, life-threatening allergy to soy, she orchestrates a plan at the wake following their friend Colin’s funeral. A flashback shows her deliberately eating something containing soy, swapping Peter’s working EpiPen for an expired one she retrieved from the trash, and removing the emergency medical kit from their car.
When Peter has an allergic reaction and the expired pen fails, he realizes what she has done.
As he continues to choke, Peter tells Marissa that she always has the emergency kit and that’s when she confirms it – that this is part of her plan to kill Peter.
With the ambulance minutes away and no way to save him, Peter dies. When questioned by police, Lia, who has learned the full extent of Peter’s manipulation, chooses to lie and protect Marissa.
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In the final scenes, Detective Alcaras visits Marissa at her home. Through his investigation, he has uncovered the connection between Carrie and Milo: a rare neurological condition called synesthesia, which both of them shared. This inherited condition convinced him of their biological relationship.
The detective tells Marissa he has pieced together the truth—that Peter caused the fatal car accident, switched the babies, and killed both Carrie and her father. He acknowledges that Marissa was unconscious during the initial switch and only learned the truth when Carrie arrived at her door.
“Although he has cracked the case, he tells Marissa that it’s actually officially closed now. Even though it shouldn’t sit right with him, he admits he feels fine and leaves”.
The series ends with the case closed not by an arrest, but by a detective’s personal sense of justice. Marissa and Milo are shown moving on, visiting Jenny and her son for a playdate, mirroring the ordinary moment that started the entire chain of events.
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