Mari Gilbert, portrayed by Amy Ryan, is a tenacious single mother managing the challenges of life in New York while raising her three daughters: Sherre, played by Thomasin McKenzie, Sarra (Oona Laurence), and the eldest, Shannan (Sarah Wisser), who has moved out. When Shannan goes missing on the day of a family dinner, Mari's growing unease quickly escalates into desperation. As days slip by with no word from her daughter, Mari and Sherre set off on a frantic search for answers.
Their quest reveals that Shannan had been living a hidden life as a sex worker in Long Island—an earth-shattering truth that only strengthens Mari's resolve to bring her home. After the police turn a deaf ear to her pleas, Mari meets with Detective Dean Bostick, who downplays her fears, believing Shannan will return. However, a dog’s discovery on the highway leads to a grave turning point in the investigation as the authorities unearth multiple bodies, pushing Commissioner Richard Dormer to confront the unsettling possibility of a serial killer stalking the area. Although Dormer assures Mari that Shannan is not among the victims, she remains steadfast in her pursuit of the truth.
As the weeks drag on and media coverage intensifies, Mari grows increasingly frustrated with the lack of attention from law enforcement. Driven into action, she tirelessly prints missing person posters and visits the police station daily, intent on raising her voice until Shannan's whereabouts are uncovered. Joined by a community of grieving families—including Lorraine (Miriam Shor) and Lynn (Ana Reeder)—and Kim, a former sex worker whose sister is among the missing, Mari finds strength in their shared anguish and determination.
Together, they take to the streets to plaster the neighborhood where Shannan was last heard from with posters of her face. The presence of Dormer in their midst ignites conflicting emotions among the families, aware of the police's insufficient response. Fueled by anger, Mari confronts Dormer, insisting that their negligence regarding the girls’ disappearances is unconscionable.
At the same time, an enigmatic informant, Joe Scalise, reaches out to Mari, hinting that Dr. Peter Hackett may be further entwined in the disappearances than anticipated. Digging deeper, Mari discovers that the security footage from the night Shannan vanished has yet to be reviewed by the authorities.
As tensions escalate, Mari boldly challenges both Dormer and Bostick about the course of the investigation, revealing her transformation from a grieving mother to a fierce advocate for justice. In the midst of this turmoil, Sherre learns that Kim has returned to a difficult lifestyle, adding to the group's stress.
As the search expands, Dormer gets caught up with Dr. Hackett, urgently seeking answers about the missing footage, but the urgent call to action lands on Bostick’s shoulders as he discloses to Dormer the chilling fact that the bodies of the missing girls have remained hidden in the same place for fifteen years. This explosive information sends shockwaves through the community, leaving Mari and the families in disbelief from the implications this has for Shannan's case.
Overwhelmed with a mix of anger and heartbreak, Mari comes to terms with the idea that if it weren't for Shannan's desperate situation, these other losses could have remained hidden. Dormer tries to placate her by insisting on sticking to evidence and facts, but Mari's pain is amplified when she learns that her daughter had been in foster care, which further complicates their family dynamics.
As time continues to wane without resolution, Mari's sense of injustice grows ever stronger. A call from Joe regarding Dr. Hackett’s sudden exit compels Mari to confront Dormer about the marshland near where Shannan’s jacket was found, demanding thorough searches that have yet to be undertaken. Dormer's initial reluctance to act pushes Mari to courageously proclaim she will publicly seek justice for her daughter.
Ultimately, Dormer’s conscience prevails as he orders the marsh to be drained, leading to the grim discovery of Shannan’s remains—an harrowing acknowledgment of the failure to act sooner. As Mari whispers a heartfelt goodbye to her daughter, she becomes an embodiment of unyielding resolve to ensure accountability.
As the anniversary vigil approaches, Mari seizes the moment to rally support not just for her daughter but for all the affected families, passionately calling for justice. Her heartfelt plea is broadcasted to national audiences, demanding the attention that has long been denied.
In a tragic turn, news outlets report that an independent autopsy reveals Shannan’s death resulted from homicide, a grim fact in light of the ongoing mystery surrounding the Long Island serial killer. This haunting reminder of the all-too-frequent injustices serves to underscore Mari's fight, underscoring her legacy of tenacity and determination.
In a poignant twist of fate, news footage revisits Mari, revealing her tragic end in 2016 following a battle with schizophrenia, while her daughter Sherre remains resolute, ensuring Mari's fight for justice continues to shine as a beacon of hope amidst the darkness.
