Casey arrives at her Vermont lake house after falling into alcoholism following her husband Len's death and career collapse.

Casey spots someone drowning in Lake Greene and rescues her neighbor Katherine Royce.

Tom and Katherine Royce arrive at Casey's dock with expensive wine to celebrate Katherine's rescue.

Katherine suddenly collapses in Casey's yard after drinking the wine Tom carefully poured for the group.

Casey visits the Royces' house and Tom dismisses her concerns, claiming Katherine has returned to New York.

Casey discovers that a recent Instagram post showing Katherine home in New York was staged and photographed by Tom.

Casey and Boone watch Tom return from a hardware store carrying tarp, rope, and a hacksaw.

Detective Wilma Anson reveals that Tom Royce is a suspect in the murders of three young women.

Casey breaks into the Royces' house and finds Katherine's cell phone hidden inside a drawer.

Tom arrives at Casey's door and demands she stop spying, warning her she does not understand the situation.

Casey dials the unknown number saved from Katherine's hidden phone and Boone's own phone immediately begins to ring.

Casey finds Katherine tied up and sick in the basement of the abandoned Fitzgerald house.

Katherine, speaking as Len, reveals the one secret only the two of them share: that Casey murdered him.

Casey kisses Katherine/Len to pull Len's soul into her own body, freeing Katherine from his possession.

Tom confesses he has been slowly poisoning Katherine; Casey uses a broken wine bottle to kill Len inside Tom's body.

Casey Fletcher retreats to her Vermont lake house after losing her husband and career to alcohol, intending to drink in isolation and avoid her past. She rescues her glamorous neighbor Katherine Royce from drowning, sparking a friendship that quickly turns alarming as Katherine's husband Tom behaves with growing suspicion. Casey teams up with neighbor Boone Conrad and detective Wilma Anson to investigate Katherine's unexplained disappearance, gathering evidence that Tom may have harmed her. Infiltrating the Royces' house and tracking strange clues, Casey uncovers a truth far darker than she expected — Katherine is possessed by the spirit of Casey's own murdered husband, Len. Forced to confront both Len's crimes and her own guilt, Casey devises a desperate plan to free Katherine by drawing his soul into herself. She ultimately defeats Tom — and Len — in a final violent confrontation and begins the slow road to healing.

The central mystery of Lake Greene unfolds.

Casey's mother Lolly makes her daily check-in call after the rescue, urging Casey to focus on recovery.

Casey spies through binoculars as Katherine makes a secret phone call that Tom quietly intercepts.

Boone Conrad visits Casey's porch but declines her offer of a drink, revealing he is in AA.

Katherine visits Casey for morning coffee and jokes that Tom would kill her before letting her leave.

Casey watches through binoculars as Katherine punches Tom in the kitchen and the two women lock eyes through the glass wall.

Casey's cousin Marnie reports that the doorman at the Royces' apartment building says Katherine has not been home.

Casey visits Boone next door and discovers he also heard a scream from across the lake that morning.

Casey downloads Tom's Mixer app and finds Katherine's location still registering at the lake house, not New York.

Casey and Boone visit a store owned by the Keene family and ask if anyone recognizes Tom Royce.

Boone confesses to Casey that he once showed up drunk to a police call, nearly shot a man, and was suspended.

Boone arrives at Casey's house after Tom's threatening visit and carries her to bed.

Casey confronts Boone about the phone call; he admits to a summer flirtation with Katherine that ended in a kiss.

Casey rows her boat alone through the raging storm toward the darkened Fitzgerald house.

Casey takes Wilma by boat to Old Stubborn and presents Len's victim trophies as evidence of his crimes.

Casey wakes on New Year's Day to find her mother, Marnie, Eli, and Boone gathered at her table.

While the main mystery builds, a web of secondary relationships and discoveries shapes the direction of Casey's investigation. Early warning signs multiply through small but significant moments — Katherine's unsettling coffee-visit joke about Tom, a secret phone call intercepted across the lake, and mounting concern from Casey's family. Casey and Boone gather key evidence together, including confirmation of the scream, the faked location data on the Mixer app, and a cautious visit to Megan Keene's grieving family. The investigation deepens emotionally as Boone shares his own story of a drunk on-duty incident, and as Casey begins to develop feelings for him that complicate her judgment. A pivotal turn arrives when Boone's hidden connection to Katherine is exposed and Casey, feeling isolated, decides she must act outside the law. The story closes months later with Casey surrounded by her rebuilt support network on New Year's Day, having finally chosen sobriety.

The relationships and clues driving the investigation.

Eli tells the group around Casey's fire that reflective surfaces were once believed to trap the souls of the dead.

Casey wakes to a sound she believes was a scream from across the lake, but her hangover makes her doubt herself.

Casey watches through binoculars as Katherine researches something on a laptop before Tom sneaks up and forces her to close it.

Casey finds searches on the Royces' computer about a poisoner who drew inspiration from Casey's own Broadway play.

Tom returns home unexpectedly while Casey is searching the house; Boone distracts him outside so Casey can escape.

Tom watches Casey back through his own binoculars from across the lake before getting in his car to confront her.

Casey grabs a kitchen knife and forces Boone out of her house, suspecting he was the one poisoning Katherine.

Casey searches online and discovers Boone was investigated for inconsistencies in his account of his wife's death.

Casey calls Wilma to report suspicions about Boone, but Wilma dismisses her, leaving Casey convinced she must act alone.

Tom reveals to Casey that he has been keeping Katherine captive because he believes she is possessed by a spirit.

Casey's suppressed memories of finding driver's licenses and locks of hair inside Len's tackle box come flooding back.

Casey remembers drugging Len with wine and antihistamines and pushing him out of the boat to drown.

Eli arrives at Casey's house during the struggle, pulls Len/Katherine away from her, and listens to Casey's full confession.

Casey ties an anchor to herself and jumps into Lake Greene; Katherine dives in and pulls her back out.

Boone tells Casey the truth about his wife's death by suicide and invites her to join him at an AA meeting.

Beneath the thriller surface of The House Across the Lake lies a series of devastating personal revelations that reframe the entire story. A campfire tale about souls trapped in reflective surfaces turns out to be literally true, and suspicious details — a stifled late-night laptop search, a poisoning method lifted from Casey's own Broadway play — quietly point toward Tom's guilt. When Boone is exposed as having lied about knowing Katherine, Casey's suspicions shift dangerously before she discovers further unsettling details about his past, leaving her convinced she must act entirely alone. The deepest revelations belong to Casey herself: long-suppressed memories of discovering Len's serial killer trophies and of pushing him from the boat finally surface with devastating force. Casey's decision to sacrifice herself to free Katherine, and Katherine's rescue of her from the lake, mark the story's emotional climax. Boone's quiet confession about his wife's suicide and his offer of an AA invitation signal that Casey is finally ready to stop running from her past.