All Her Fault: Sarah Snook’s shocking twist ending explained in finale

All Her Fault, the Peacock thriller starring Sarah Snook, delivered a shocking finale. The limited series premiered on Nov. 6, 2025. The finale reveals four major twists involving murder, baby swaps, and dark secrets. Marissa Irvine (played by Snook) discovers her entire life was built on lies.

How the Shocking Ending Unravels

The final episode of All Her Fault shatters Marissa’s world completely. Peter confesses a devastating secret during their confrontation. Six years prior, the couple survived a horrific car accident. Another mother named Josephine Murphy (known as Carrie, played by Sophia Lillis) was driving the other vehicle. Peter’s newborn died in the wreck. Josephine’s baby survived.

“I switched the newborns after the accident because I thought Carrie was dead.” Peter told Marissa.

Marissa realizes she’s been raising someone else’s biological child. Carrie/Josephine discovered this truth years later through synesthesia. Both she and Milo share this rare neurological condition. She grew obsessed with reclaiming her son. She posed as Jenny’s nanny to get close to Milo. She orchestrated the kidnapping that started the entire crisis.

Peter’s Dark Crimes Revealed

Carrie arrives at the family’s home with a gun to warn Marissa. She plans to expose Peter’s twisted cover-up. But Peter shoots her first. This wasn’t an act of self-defense. Carrie was begging for her life. Peter had also murdered Carrie’s father earlier. Her father possessed a recording that proved Peter killed him.

Marissa faces an impossible situation. Peter explains that going to police means losing Milo forever. She knows he’s right. Social services would take custody. She could never prove Milo was safer with her. She decides to stay silent and plan Peter’s death.

A Calculated Murder by Poisoning

Marissa makes her move weeks later at Colin’s funeral. She removes the emergency medical kit from their car. She replaces Peter’s active EpiPen with an expired one. She deliberately eats soy-filled food before kissing him. Peter suffers anaphylactic shock.

Key Plot Points	Details
The Car Accident	Six years before the series. Peter’s baby dies. Josephine’s survives. Peter switches them.
Carrie’s Discovery	She shares synesthesia with Milo. She learns he’s her biological son.
The Kidnapping	Carrie and her father take Milo. They demand ransom and truth.
Peter’s Murder Spree	He kills Carrie’s father. He shoots Carrie. He lies to everyone.
Marissa’s Revenge	She poisons him with soy at the funeral. Peter dies from anaphylaxis.

Marissa watches him with chilling calm as the anaphylaxis progresses. In his final moments, Peter looks at her with realization. He knows she poisoned him deliberately. She tells him: “That’s true — I manage your allergy.” She watches him die without helping.

Detective Alcaras Lets Her Walk Free

Detective Alcaras eventually pieces together what happened. He discovers that Marissa murdered her husband. He finds her afterward and explains his discovery. But he also explains his decision. He tells her he feels “fine” not investigating Peter’s death further.

Alcaras has compromised his own morality to protect his son. He understands Marissa’s motivation. He knows Peter was a monster. He chooses to let her walk free. She escapes full accountability for her crime.

What Happens to Marissa and Milo?

All Her Fault concludes with a hopeful final scene. Marissa and Jenny sit in the backyard. They watch their sons play together with bubbles. Both women sip wine and smile. They’ve found refuge in their friendship. They’ve created a new family without the men who threatened them.

Sarah Snook revealed that Marissa faces years of fallout. Milo will eventually learn Peter isn’t his biological father. That trauma will compound his kidnapping trauma. Marissa will struggle with whether to tell him the truth about Peter’s darkness. But for now, they’re safe. Marissa has regained control of her narrative through murder.

Why This Ending Changed from the Book

The novel by Andrea Mara handles Peter’s death differently. In the book, it’s just a quiet mention. A news article reports Peter died from allergies while traveling abroad. Creator Megan Gallagher chose to expand this. She made Marissa actively commit the murder. Snook approved because Marissa needed the final say. She needed to reclaim her agency after months of chaos.

The television adaptation set in Chicago deepens the story beyond the Dublin-based novel. It explores themes of motherhood, privilege, and how wealth doesn’t protect families from devastation. Marissa breaks free by poisoning her husband. She chooses her son over the law.

What Critics Said About the Twist

Critics praised Snook’s performance throughout the ordeal. The Guardian called it “fantastically well done.” NPR noted that Snook plays Marissa with “remarkable restraint.” The Hollywood Reporter said she’s “excellent at playing tremulous and overwhelming feeling.” She anchors the chaos with emotional authenticity.

Is There a Season Two?

All Her Fault is a limited series with eight episodes. It concludes with Marissa free and happy. Peacock hasn’t announced season two plans yet. The finale provides closure. Marissa achieved her goal: protecting Milo by any means. She’s made peace with Jenny. She’s escaped legal consequences. There’s no obvious cliffhanger demanding continuation.

The streaming landscape suggests Peacock might develop Gallagher’s next thriller instead. She created a complex narrative in All Her Fault. The twist ending satisfied fans. The show proved Snook can anchor a thriller series after Succession.

Where to Watch All Her Fault

All Her Fault streams exclusively on Peacock in the United States. All eight episodes are available now. In the United Kingdom, the show streams on NOW with an Entertainment Membership. The series runs approximately 50 minutes per episode. You’ll finish watching in under seven hours total.

Streaming Details by Region

United States: Peacock Premium or Peacock Premium Plus. United Kingdom: NOW TV Sky exclusive. Australia: BINGE. New Zealand: TVNZ+ (free). The series has high ratings on Rotten Tomatoes (77%) despite divided reviews.

The Cast and Creative Team

Starring Sarah Snook as Marissa Irvine, Dakota Fanning as Jenny Kaminski, Jake Lacy as Peter Irvine, and Sophia Lillis as Carrie Finch/Josephine Murphy. Michael Peña plays Detective Alcaras. The series was created by Megan Gallagher and adapted from Andrea Mara’s 2021 bestselling novel.

Sources

• Variety – ‘All Her Fault’ Twist Ending: Sarah Snook on Why Marissa Kills Peter
• People – All Her Fault Ending Explained: What Happened to Carrie and Milo?
• The Guardian – All Her Fault review – Sarah Snook’s terrifying thriller