The eighth and final episode of the hit drama mystery series All Her Fault brings the audience up to speed about what actually transpired over the past six years, and in my opinion, does enough to pay off its twists and leave us feeling satisfied about the final choices made before the credits roll one last time.
We left off at the end of episode 7 with a major cliffhanger: Carrie enters the Irvine home with a gun pointed at the family. Colin plays hero and tries to get the gun from her hands, but it goes off in the struggle and kills Colin. Carrie reveals to Marissa that she's actually Josephine Murphy, who Marissa thought had died in the car accident. And things get too real for Peter when she hints at how Peter is the true villain in the story. And when Peter learns that Josephine has a voice recording with the truth, he forcefully takes the gun and murders her.
With Marissa having enough of an idea, she demands Peter tell her exactly what he did the night of the car accident, and we're brought into a flashback montage of the accident intercut with hints dropped in the series (like the scar we saw on Marissa's shoulder in an earlier episode). Marissa finally knows the truth: her son died in the car accident, and Peter switched Milo for Noah. Peter desperately tries to defend himself, but Marissa now knows Milo was kidnapped because of Peter.
When the detectives arrive at the home, Marissa lets Peter take the lead because she knows deep down that if they learn Milo isn't biologically theirs, he could be taken from them. Marissa hears all the lies that fall so easily from Peter's mouth, and we can see her brain spinning about what to do next -- and I'm not convinced the detectives believe him either. Once she's able to walk away, she plays the recording on Carrie/Josephine's phone: it was Peter who went to the motel room, killed Rob, and put Milo in the trunk of the car. And out of fear of what Peter could do next, she reluctantly gives him the recording to destroy.
Marissa's final plan to save Milo
Even though the detectives aren't convinced they have the full story, the case in Josephine's murder is officially closed. Though Peter is relieved, thinking he's off the hook for his crimes, Marissa is still (obviously) terrified of what Peter's shown he's capable of. She fully confides in Jenny, who validates that Milo is still Marissa's son and that they need to keep the secreted protected. But, they both know Peter can't stay in their lives.
As the Irvine's get ready for Colin's wake, Detective Alcaras is continuing to investigate a lead in the case that he stumbled into: Josephine and Milo likely both have synesthesia, a neurodivergent trait that is often inherited. He's on track to realize about what actually happened the night of the car accident six years ago. He goes on to follow up with the actual paramedic who was on the scene, and he confirms Marissa was unconscious.
At the wake, Marissa and Peter walk down to the basement to talk, and to Peter's surprise, Marissa starts passionately kissing him. Within a few seconds, Peter can feel the beginning of an allergy attack (we learn early in the season that he's severely allergic to soy), and they both run upstairs to get his EpiPen. When the pen doesn't go off, Peter realizes it expired three years ago. Leah runs for the car emergency kit and calls for help, but the kit is missing!
Marissa slyly lets Peter know that this allergy attack was no accident, but she did everything in her power to make his death look like it was. And with the ambulance eight minutes away, Peter dies from anaphylaxis before he's able to get help. The detectives question Leah to ask whether Marissa had purposefully eaten the soy, and she verifies that Marissa forgot to read the labels (even though we know she did).
The end of a nightmare
Detective Alcaras visits Marissa at the new house she's moved into with Milo, and he starts off by letting on that he knows Milo is actually Josephine's biological son. He explains that Milo and Josephine both have synesthesia, which tipped him off to what actually happened. He walks through exactly what he thinks actually led to the kidnapping and the murders of Josephine and Peter, and he's correct every step of the way.
Even though it's not the "legal" thing to do, he acknowledges that it feels right. The case is closed and he doesn't want to reopen it. He knows that the best case scenario now is for Marissa and Milo to stay together as a family. And in the final scene of the episode, Marissa and Jenny sip wine together while Milo and Jacob play. Their plan worked.
All eight episodes of All Her Fault are currently available to stream on Peacock.