Isa receives Kate's 'I need you' text and lies to her partner Owen to travel to Salten with infant Freya.

Kate gathers the women together and shows them a newspaper headline about a human bone found in the Reach.

The four women agree to tell police they know nothing about the body and plan to attend the alumnae dinner as cover.

A dead sheep appears outside Tide Mill, and Isa secretly finds a threatening note hidden in Kate's coat pocket.

The women attend the Salten House alumnae dinner and are met with cold gossip and hostile receptions from former classmates.

A drunk Luc confronts the women on the beach after the dinner, shouting that those responsible for the body stand before him.

The women recall arriving at Tide Mill as teenagers to find Ambrose dead in his chair, holding a suicide note and heroin kit.

The women return to London and police officially identify the remains in the Reach as belonging to Ambrose.

Each of the three women receives an anonymous envelope of photocopied nude drawings of herself made by Ambrose.

Owen discovers the drawings and confronts Isa; after she lies and they fight, Isa walks out and returns to Salten with Freya.

Kate admits she has been paying a blackmailer for years and kept it secret from her friends to protect them.

Mary Wren tells Isa that Ambrose died from an oral heroin overdose and that Kate and Luc had been sleeping together.

The women confront Kate, who confesses she poured heroin into Ambrose's wine to stop him from separating her from Luc.

A fallen lamp sets Tide Mill on fire; Luc drops Freya safely to Isa from a window before dying, and Kate rushes inside after him.

Isa realizes Luc was Ambrose's true killer; the three surviving friends vow to maintain a new lie to protect Kate's memory.

When Isa receives Kate's urgent text, she lies to her partner Owen and travels to the coastal village of Salten, where Kate reveals that a human bone has been found on the beach near Tide Mill. The four women agree to maintain their seventeen-year-old story — that they know nothing — and use an alumnae dinner as cover for their presence in Salten. Threats begin to surface: a dead sheep, a menacing note, and a drunken confrontation from Kate's stepbrother Luc. After returning to London, the women each receive anonymous envelopes of Ambrose's compromising drawings, and when Owen discovers them, Isa abandons him and returns to Salten. There, Kate's secret blackmail is exposed, Mary Wren delivers a damning revelation about the true nature of Ambrose's death, and Kate finally confesses to poisoning her own father. The Tide Mill burns to the ground, killing both Kate and Luc, and Isa is left to piece together the truth and forge a new lie to protect her dead friend's memory.

The central mystery of Ambrose's hidden death and its devastating unraveling.

At fifteen, Isa meets Kate and Thea on the train to Salten House and accepts their invitation to join them.

Isa lies to protect Thea from punishment for smelling of cigarettes; Kate and Thea formally welcome Isa into the Lying Game.

The girls sneak out at night and Kate explains the Lying Game's rules over contraband whiskey on the fire escape.

Ambrose draws Isa's portrait in art class and tells her she is braver and stronger than she believes.

During the girls' first visit to Tide Mill, Isa meets Luc Rochefort, Kate's dark-haired, golden-eyed 'stepbrother', for the first time.

Isa spreads a malicious lie about Thea and shy Mark Wren on the school bus and immediately realizes she cannot take it back.

Mary Wren publicly threatens Isa in the village pub for spreading damaging rumors about her son Mark.

Kate explains to Isa how Luc was forcibly returned to France after Ambrose's disappearance, where he suffered serious abuse.

Isa recalls kissing Luc on the jetty while he drowsed; he startled awake, furious, and shoved her sharply away.

The four girls take turns digging a grave on the shore of the Reach and bury Ambrose's body in the wet sand.

Miss Weatherby confronts the girls with Ambrose's nude drawings and confiscated contraband from their rooms, announcing their expulsion.

The four friends gather at a crematorium to support Isa after her mother's death — their first reunion since being expelled from Salten.

Thea reluctantly shares that Ambrose once confided he had a difficult decision to make because something was 'all wrong.'

Isa searches Kate's room while Kate is out and finds Ambrose's hidden suicide note, rereading it with growing suspicion.

Isa decides to remain with Owen for Freya's sake, knowing she does not love him but determining to try.

At fifteen, Isa falls in with Kate, Thea, and Fatima at Salten House boarding school, bonding over contraband whiskey, shared lies, and the thrill of their secret Lying Game. Their weekends at Tide Mill under the permissive watch of Ambrose — Kate's artist father — gave them the closest thing to a real family any of them had, but also planted the seeds of a crime. When the girls secretly buried Ambrose's body on the shore, they made a pact that would follow them for the rest of their lives. Expulsion from Salten scattered them across the world, and they did not fully reunite until grief drew them together at Isa's mother's funeral. Seventeen years later, as the investigation into Ambrose's remains intensifies, long-buried clues begin to resurface — Thea's memory of a cryptic warning from Ambrose, a hidden suicide note discovered in Kate's room — forcing the women to reckon with what they actually buried that night. The pool concludes with Isa's quiet, bittersweet decision to stay with Owen for Freya's sake, even knowing she does not love him.

The boarding school bonds and buried secrets that shaped four women's lives.

Kate confides to Isa that Tide Mill is built on shifting sand and the tide will eventually sweep it away entirely.

Fatima arrives at Tide Mill having embraced Islam; she now wears a hijab and has completely given up alcohol.

Fatima catches Isa lying about how much she thinks of Luc, revealing each woman's distinct involuntary physical tell for deception.

Isa finds a drawing of a girl hidden in a Tide Mill floorboard — the face scribbled out and eyes poked through — before the wind carries it into the Reach.

Isa encounters a hostile, transformed Luc at the Salten post office, who insists Kate knew the horrors he was being returned to in France.

On the walk to the alumnae dinner, the women spot a police forensics tent erected directly above where Ambrose is buried.

Miss Weatherby apologizes to Isa at the alumnae dinner for how the school handled the girls' expulsion and invites her to send Freya to Salten someday.

Thea vomits outside after the dinner; Fatima confronts her bluntly about her drinking and refusal to eat anything.

Luc sends Isa a bouquet of red roses with an apology note after their post office confrontation; Isa throws the flowers away.

Owen and Isa fight bitterly over his exclusion from caring for Freya; Isa fears that if she opens up she will confess everything.

Luc meets Isa at the Salten pub, apologizes for his past behavior, and kisses her on the walk back to Tide Mill, saying he should have chosen her.

Luc and Isa nearly become intimate inside Tide Mill until they both notice Kate standing silently on the staircase, watching them.

The three women meet at Hampton's Lee and Fatima explains that an oral heroin overdose would be nearly impossible to self-administer.

Isa wakes in the night to the voices of Kate and Luc arguing overhead in Tide Mill, then smells burning paraffin and hears crackling fire.

In the hospital, Isa realizes that Luc — not Kate — poisoned Ambrose, and that Mary Wren was the blackmailer all along.

Beneath the main plot runs a quieter current of personal fractures and small but pivotal discoveries that slowly expose the truth about Ambrose's death and the women's own natures. From the moment Fatima catches Isa in a lie about Luc, the women's physical tells — the unconscious betrayals of the body — signal that their old rules of honesty with each other are already breaking down. Threatening signs accumulate: a defaced drawing lost to the tide, a hostile encounter with Luc, a forensics tent glimpsed on the way to dinner, and the gradual erosion of Isa's relationship with Owen. The second return to Salten brings emotional collapse as Luc and Isa act on their long-suppressed attraction, only to find Kate watching from the shadows. At Hampton's Lee, Fatima's medical expertise turns the group's suspicion toward Kate, but the fire reveals a deeper truth. The pool closes with Isa's hospital revelation that untangles seventeen years of misdirection and finally names the real killer.