Helen, Ciro, and Freddy are on a boat. Freddy goes swimming, giving Helen a chance to speak privately with Ciro, who notices a bruise on her arm from where Lorna grabbed her.

Helen recalls how she and Lorna orchestrated a blackmail scheme, using Sarah’s necklace to demand €10 million from Richard and Marcus by the anniversary of Sarah’s death, threatening to release further incriminating information to the police. Helen sent the necklace to Ciro in Naples and asked him to then mail it to the villa. Lorna was supposed to collect the money and meet Ciro, who would drive her to Naples.

Helen asks if Lorna met Ciro the previous night, but Ciro says she never arrived. Helen notices a deep cut on Ciro’s hand, which he claims is from fishing, but Helen suspects it is from a knife or glass. They kiss when Freddy is out of sight.

A New York Post article from February 27, 1993, reports that Italian authorities have agreed to release Sarah Lingate’s body to her family after a seven-month delay. The family’s lawyer claims the delay was a form of police harassment. An anonymous source suggests the police held the body because the family intended to cremate it, which would destroy any potential evidence. The article references rumors that Richard had been violent with Sarah on the night of her death and that she had injured him earlier that day.

In Los Angeles, the family prepares for the trip to Italy. Sarah reflects on Richard’s growing paranoia. After a long period of writer’s block following the birth of their daughter, Helen, Sarah has secretly written a new play titled Saltwater. She asks Richard to read it.

Sarah recalls her romance with Richard in New York, where she was a rising playwright and he was a wealthy literary host. After they married and moved to Los Angeles to attend to Richard’s dying father, Richard became controlling, and Sarah felt trapped.

When Richard reads the play, he is enraged, realizing it is a depiction of their family. He accuses her of exposing them to gossip and damaging their name. When Sarah asks what he will do if she produces it anyway, he threatens to sue her and ensure she loses everything. Sarah thinks to herself that she has already lost everything, which makes her more dangerous.

Lorna, Helen, and Freddy have drinks at the villa. Lorna is unsettled by Helen’s detached calm. Richard announces they are all having dinner on Stan’s yacht. Startled, Lorna cuts her thumb with a knife.

A doctor comes to give Lorna stitches. While alone with her, Marcus asks Lorna to listen for anything Stan says about Sarah at dinner. He says he may need her to go to shore later, and Lorna knows he is debating whether to pay the blackmail money. He also asks Lorna to reassure Naomi that there is no affair between them.

Eighteen hours after Lorna’s disappearance, Helen searches Lorna’s room and finds most of her belongings are still there, including her passport. In Lorna’s carry-on bag, Helen discovers an unopened pregnancy test just as Naomi enters the room and sees it.

At dinner, the family shows no urgency about Lorna’s disappearance. Richard tells her they have people looking into it and instructs her to be quiet. Suspicious of their lack of concern, Helen goes back to Lorna’s room and finds her laptop, which has been completely wiped. Realizing her father and uncle must have done it, she frantically searches the room and finds a manila envelope hidden under the mattress. Inside is a copy of her mother’s unpublished play, Saltwater.

A Los Angeles Times article from November 2007, 15 years after Sarah Lingate’s death, reports that two new anonymous sources have emerged. One source claims to have seen Sarah, injured, with an unidentified man near the family’s villa on the night she died. A second source, who was at the same dinner party, claims to have overheard Richard complaining that Sarah had tried to kill him earlier that day. The article quotes Stan from an older interview in which he calls for the case to be reopened.

Sarah’s agent calls to inform her that Richard’s lawyers have sent a cease-and-desist letter to stop the production of her play, and the agency may drop her as a client. Sarah, Richard, Marcus, and Naomi go spearfishing from a small boat. Sarah fantasizes about Richard dying. When a large tuna appears, Sarah fires her speargun. The fish moves, and the spear hits Richard in the calf. He screams that she shot him on purpose, while she insists it was an accident. They get him back to shore, and a doctor gives him almost 20 stitches. 

Sarah separates from the group to sit alone at a café. Marcus joins her and attempts to placate her by calling their marital problems a “phase.” Sarah tells him she wants a divorce. Marcus’s demeanor shifts, and he warns her that their divorce will not be a private matter and will inevitably create conflict between him and Richard.

Helen reflects on discovering her mother’s identity through her plays, which a professor gave her in college. As she reads Saltwater, she recognizes disturbing parallels to her own family, providing a clear motive for them to have killed her mother and possibly Lorna.

The next morning, Helen goes to the Marina Grande, intending to check for surveillance footage of Lorna. At the funicular station, she spots a woman she mistakes for Lorna and follows her down to the lower terminal. When she confronts the woman, she realizes her mistake.

Stan appears, revealing he followed Helen. He is distraught and explains that Lorna had been compiling a dossier on the Lingates, claiming she knew who killed Sarah. Stan promised to pay Lorna $500,000 for the information. Stan confesses his unrequited love for Sarah and tells Helen, for the first time in her life, that her mother loved her very much. Helen decides to confide in Stan.

The Lingates, Freddy, and Lorna board Stan’s yacht for dinner. The scene reminds Lorna of her past as a party girl, and she sees three young women: Sasha, Martina, and Giulia. Lorna, Helen, and Freddy sit with the women, and Naomi joins them, visibly inebriated. Stan shows Marcus and Richard his helicopter and office, neither of which impress Marcus.

During dinner, Stan provokes Marcus by insinuating he is having an affair with Lorna. Marcus retorts that Stan used to chase after Sarah. The exchange escalates until Naomi, upset, leaves the table, and Marcus follows her. Stan notes Sarah’s necklace, which Helen is wearing. After a tense meal, the group goes to a party at Taverna Anema e Core.

Helen agrees to help Stan expose her family, though she doubts that any external attack on the Lingate family could succeed. Stan gives her a ride on his boat, and they encounter several police boats in the secluded inlet where Lorna nearly drowned Helen.

They watch as police divers pull a net from the water containing a body. From the matted dark hair and thin limbs, Helen knows it is Lorna. Staring at the body and thinking about her mother’s play, Helen realizes that her family is capable of killing Lorna.

Helen and Freddy drink and dance at Anema e Core, and Lorna wishes she could drink, too. Marcus, Richard, and Naomi left earlier, but Marcus texts Lorna that he is returning with the money. While watching Helen, Lorna sees her slip away into a dark corner and kiss Ciro. Lorna realizes Helen has secrets and backup plans of her own.

Helen finds Lorna in the bathroom and confirms the plan is moving forward. Back in the club, Stan appears and passes Lorna the bag Marcus left for her. He separately pressures her for the promised dossier on the Lingates, and she tells him he must wire her the payment first.

Lorna leaves with the bag and takes a motor scooter ride to the marina. While walking down a deserted street toward the boat slips, she hears a group of men following her. Just as she prepares to flee, a familiar voice calls her name from the darkness. The group of men walks past her without incident. Lorna is left alone on the dark street and realizes she is in danger.