When Mari Gilbert’s daughter disappears, police inaction drives her own investigation into the gated Long Island community where Shannan was last seen. Her search brings attention to over a dozen murdered sex workers.
Mari Gilbert (Amy Ryan)’s eldest daughter Shannan goes missing, and when the police don’t take it seriously, she takes it upon herself to find answers. She discovers Shannan was working as a prostitute, and due to her raising a ruckus, the police find multiple bodies buried in Long Island, all of young sex workers.
Despite the awareness of a serial killer, the police say that Shannan’s disappearance is separate and unrelated, despite Mari’s protests. A year later, Mari demands that Commissioner Dormer (Gabriel Byrne) drain the marsh near where the girls were found. He finally agrees to do so, and Shannan’s body is finally found. Mari goes on the news and demands that the police be held accountable for letting down the girls, and takes her other daughters home.
Post-script explains that the autopsy confirmed Shannan was indeed one of the murder victims, the killer of which is still at large. In 2016, Mari’s youngest daughter had a psychotic schizophrenic episode and fatally stabbed Mari. Mari’s other daughter Sherre continues to be an advocate to this day.
Mari Gilbert (Amy Ryan) is a blue collar working single mother working multiple jobs in New York. She has three daughters, oldest Shannan (Sarah Wisser) who is older and moved out of the house, middle daughter Sherre (Thomasin McKenzie), and reclusive youngest daughter Sarra (Oona Laurence), who has psychiatric issues and is on mood stabilizers. Shannan plans on coming home to have dinner with the family one night, but never shows up.
After a few days, Mari and Sherre grow worried when no one hears from Shannan, and when the police don’t do anything, they begin searching themselves, getting Shannan’s phone records leading them to her boyfriend and her driver, eventually discovering that Shannan was working as a prostitute in Long Island and that after a bad gig she was hysterical and made a 911 call. Mari meets with local detective Dean Bostick (Dean Winters), who believes Shannon isn’t missing and will turn up, despite Mari’s insistence she is indeed missing.
An officer lets his dog out to go the bathroom on the side of the highway, and it picks up a scent off the parkway, and knowing there is a missing person, the police finds multiple bodies buried. Commissioner Richard Dormer (Gabriel Byrne) tells the media that he can’t make any conclusions, but Bostick believes it must be a serial killer. Dormer meets with Mari and tells her that none of the bodies were Shannan, but that the search for more bodies will continue. He also tells Mari not to speak to the media.
As Shannon continues to not be found, Mari feels ignored and slighted by the police who are not getting any results even as the serial killer story begins gaining traction in the media. Mari takes matters into her own hands, printing missing posters and and buying nice clothes to go down to the police department and puts up a poster for every day Shannan has been missing, promising to raise hell.
Mari reluctantly agrees to participate in a vigil that the other victims’s families are participating in. The Gilbert family meets at a restaurant with the other grieving mothers and siblings, including mothers Lorraine (Miriam Shor) and Lynn (Ana Reeder) and the wild Kim (Lola Kirke), a former working girl and a sister of one of the victims. The other mothers are happy that their missing girls are finally getting attention, but Mari angrily tells them all they need to make noise, and that the police only cared about their girls by accident.
Mari and the family members cover the private neighborhood of Shannan’s last call with missing posters, causing the neighborhood to complain and Dormer to show up within ten minutes, which Mari finds ironic since when Shannan called it took the police an hour to show up. Dormer accuses Mari of just being guilty that Shannan grew up in foster care, and Mari tells Dormer she knows he bungled one of his previous cases.
Mari gets a call from Joe Scalise (Kevin Corrigan), a local who has sensitive information for her and meets her in secret. He has suspicions of Dr. Peter Hackett (Reed Birney) and says the police have stopped returning his calls. A doctor called Mari the day Shannan went missing, so she thinks this adds up. Mari goes to Dormer and Bostick – Bostick is annoyed that she thinks she’s an investigator, but Dormer insists on following up. Meanwhile, Sherre discovers that Kim has gone back to work on Craig’s List.
The police expand the search to the beach, and Dormer meets and interviews Dr. Hackett and questions why he didn’t give the police the security footage from his home the night Shannan went missing. Meanwhile, Joe believes that his attack on Shannan became complicated and so that’s why she wasn’t buried with the other girls. Bostick tells Dormer the bodies have been buried there for fifteen years, and subsequently, Dormer goes on TV and tells the media that Shannan, while also considered a missing person, is not considered related to the serial killer, not fitting the profile of the other victims location, and is not part of the serial killer investigation.
Mari is furious, since none of the other girls would have been found if not for Shannan and she shares so much in common with the other victims, but Dormer tells her he’s only going by the evidence. Sherre overhears the conversation, and confronts her mother about sending Shannan to foster care – she thought the state took Shannan away. Mari explains that Shannan was bipolar and endangering herself and she didn’t know what to do. Later, after the vigil, Mari bids a tearful goodbye to Kim.
One year passes. Joe calls Mari and tells her Dr. Hackett is moving away. Mari goes to Dormer, who is about to be retired, and wants to know why they haven’t searched the nearby marsh. She wants to know why Shannan’s jacket hasn’t been tested – Dormer tells her since Shannan wasn’t officially missing at the time it was lost. Mari tells him she’s going to go on the news… but Dormer thinks it over and has the marsh drained before he’s out of his job.
When the marsh is finally drained, Shannan’s body is found. Mari and Sherre go to see Shannan’s remains, and Mari whispers to Shannan that she’s taking her home. The families are in town for the one year anniversary vigil and support Mari as she goes on television and demands accountability for herself and the police for letting down these girls. She then tells her girls it’s time to go home.
Post-script says that an independent autopsy contradicted the police and confirmed that Shannan died of homicidal strangulation, and there was no evidence of drugs in her system. Ten to sixteen victims have been attributed to the Long Island serial killer, who has still not been caught. Phone records confirmed Hackett called Mari, who filed a wrongful death suit against him. Real news footage of Mari plays, then the post-script explains that in 2016, after going off her schizophrenia medication, Sarra had a psychotic episode. Mari tried to intervene and was killed. Sherre remains an advocate fighting for justice.